DIY Refugium for the Seahorse System

I. Introduction to Refugia

What is a refugium?

A refugium is a body of water, separate from the main tank, which shares a water source with the main tank. It is not unlike a sump; however, the very definition of "refugium" implies that it is used as an area of shelter.

Why add a refugium?

I first became interested in the idea of adding a refugium to my seahorse tank because of a successful freshwater tank that I had planted. I noticed that water in the freshwater tank was always crystal clear and that the tank required very little maintenance. It seemed to me that using plants in an aquarium created a nice, circular kind of ecosystem that could enhance the water quality for my seahorses.

A refugium can provide a number of benefits to your tank:

1. It can provide an area of shelter for fauna and flora that would otherwise become food for the inhabitants of your main tank. Copepods, amphipods, Mysid shrimp and other critters can reproduce without predation. If you build an overhead refugium with a gravity return, some of the critters will eventually flow to the main tank and provide an additional food source for your seahorses.

2. It can provide a place to grow macroalgaes to export nutrients from your main tank. Caulerpa and other macroalgaes consume ammonia/nitrates and phosphates that accumulate due to overstocking or overfeeding. Placing these in your refugium will allow them to grow without predation by herbivorous inhabitants.

3. If can assist in maintaining pH levels. If you run your refugium on a reverse photoperiod from your main tank, you won't get the pH fluctuations that occur during the light/dark cycle.

4. It can be used for equipment that you don't want hanging from your display tank such as a UV sterilizer.

5. It can increase your water volume.

The way you design your refugium is largely dependent on what you wish to accomplish with it.

I built my refugium to achieve many of these goals, and more. My seahorse tank is, by any measure, overstocked. I wanted the filtration benefits I'd read about. I was also increasing my herd size significantly and wanted to increase the water volume without buying a new tank. In addition, I wanted the refugium to provide a place to raise fry where there would be a large population of copepods and other critters, and which would share a water source with their birthing tank. The reverse photoperiod would eliminate pH fluctuation that may be detrimental to fry survival rates.

II. A Primer on Plumbing:

The concept of plumbing was probably the most difficult aspect of the refugium for me to understand. If you are lucky enough to have purchased a pre-drilled tank, you probably have no difficulty understanding how it works. I didn't get it. I did recognize, from reading the online aquatic message boards, that preventing spills was a major consideration in plumbing tanks. I eventually spent weeks poring over articles, looking at various pieces of equipment and diagrams before I really understood the concept. If you are unfamiliar with aquarium plumbing, don't expect to understand all of these concepts as you read along. I'd suggest reading this section first from beginning to end, as it is difficult to comprehend the purpose of the various parts until you see how it all fits together.

So you know that the water must go up to the refugium and back down to the main tank, but how?

There are three essential parts of the plumbed aquarium system:

1. The overflow
2. The pump
3. The lines (pipe and fittings)

The Overflow:

I first thought that I'd need two pumps — one to pump up and the other down. Wrong. It is virtually impossible to equalize the flow of water through the system, plus you run the chance that one or both pumps will fail and you end up overflowing one tank or the other. You avoid this by using one pump to go up and an overflow to go back down, making the flow down dependent on the amount of water being pumped up.

The overflow system is the same thing used to plumb a tank and a sump; however, a sump is generally placed below the main tank, while a refugium is above it. Here's a nice schematic to show you how the overflow system works: http://www.sasala.com/fish/ref/overflow/overflow.html. As you look at the drawings, imagine the top tank as a small refugium and the bottom tank as the larger display tank. It works the same but the tank sizes are reversed from those shown.

There are two ways to get water back down to your tank via gravity. One type of overflow uses a hang-on box, and the other requires a drilled tank.

Hang-on overflows: This device consists of two acrylic boxes. One hangs inside the highest of your two plumbed tanks, and has slots for water to flow into it while (hopefully) keeping fish and other critters out. It also has a box that hangs on the outside of the tank, which is attached to a hose going down to the lower tank. Water moves from the inside box to the outside box by use of a clear acrylic "U-tube." Siphon keeps the water flowing from the inside-tank box to the outside-tank box. If the siphon breaks (e.g., algae or a snail gets in the u-tube), you have a real mess on your hands. Water will stop draining from the top tank to the bottom tank. At the same time, your pump will continue to pump water from lower tank up, possibly emptying the lower tank and overflowing the higher tank onto the floor and burning itself out in the process when it runs dry.

That said, a lot of people use hang-on overflows without incident. CPR and Tide pool both make overflow boxes worth looking at if you aren't into DIY and you don't want to drill your tank.

Drilled overflows: These consist of a small corner of the higher tank being sectioned off using a watertight acrylic box. Inside the box is one or more standpipes, mounted in holes drilled in the bottom of the tank. Water flows from the main chamber of the tank into the overflow chamber and then down the standpipe into the lower tank. (The All-Glass Aquarium site has a nice demonstration of how their overflows work: http://www.all-glass.com/products/aquariums/twinflo_demo.html).

It's a pretty nifty design but if you don't buy a tank with built-in overflows, you'll have to get the tank drilled to build it yourself. Drilling a tank is expensive, if you can find someone to do it at all. And it requires that you decide where to place the overflow-it must be positioned high enough that water only flows down at a certain level. If the water gets below this level (as in the case of your pump losing power and not sending water up to the highest tank), it stops draining down to the lower tank. If you place your return too low on the higher tank you may end up draining it completely. It's not rocket science though, and a little common sense goes a long way. In my opinion, a drilled overflow is definitely the best way to go.

Selecting a Pump:

Generally, your refugium needn't have too much water flow. I think 2-3 times the volume of the main tank is sufficient (e.g., if you have a 30-gallon tank, you'll want a pump that can handle 60–100 gallons per hour). You can't, however, just look at the rating on the pump to determine which one to buy, because these rates assume that the pump is taking from and returning water to the same source. If the water has to travel through a pipe, up and around a corner, flow rates will be reduced.

Movement of water from a main source through pipes or tubes is called "head." Some pump companies publish stats of what their pumps will deliver at 0 head. There is a big difference between this and what that same pump will deliver at the head it will have to deal with in your system. The distance that the water must travel up is called lift, and this distance reduces flow rates considerably. Another consideration is friction-unless you can pump straight up to your refugium, there will generally have to be elbows in your pipe, which further affect the stated GPH. A rough rule of thumb is that any 45-degree elbow will equal one foot of extra head, and a 90-degree elbow two feet. Total head will equal lift plus friction through any elbows. Unless you understand these concepts, the published flow rates can be very misleading. Flying Fish Express has a good chart to determine the flow rates of different pumps at a given head: http://ffexpress.com/drygoods/powerheads.htm.

In choosing a pump, you must also consider the size of your overflow. Most pre-drilled overflows are rated in terms of the GPH that they will handle. It is important to choose a pump that matches the capacity of your overflow. This is not as important if you build your own overflow because you can then determine your return rate by the size of the standpipe or the number of holes you drill in it.

You also need to decide whether the pump should be a submersible (sit in the water) or an external (sits outside the tank). Submersible pumps are cheaper, quieter, and easier to install, but may add heat to the water.

Lines:

Okay, you have chosen a pump and an overflow method. All that's left to do is run the plumbing from the refugium to the main tank, and from the tank up to the refugium.

You can use clear vinyl tubing for this and it probably works fine, but I like PVC. PVC and fittings are like tinker toys for fish geeks.

PVC is poly-vinyl chloride pipe-a common plastic pipe used for cold water distribution in plumbing. PVC is available at any local hardware store and is easy to work with — it can be cut and snapped using a cheap hacksaw. You cut the lengths you need and then connect them with various fittings. Although it is not particularly attractive (white with stamped printing), it quickly becomes encrusted with coralline algae. For some reason, coralline really likes to grow on some synthetic substances. PVC pipe is also cheap — less than $2 for an 8' length.

PVC pipe comes in various diameters. For most aquarium purposes, you can use either 1/2" or 3/4" (or larger, if you have a very large aquarium). Your PVC, bulkhead and fittings should all be the same size. "Fittings" are plastic pieces in various shapes that you can use to curve ("U"s, "L"s), split ("T"s) or just connect the PVC.

The most important fitting for aquarium purposes is the bulkhead. You probably won't have a lot of luck finding bulkheads at your local hardware store, but many fish stores do carry them, or you can buy them online. A bulkhead is a three-part fitting that resembles a large nut and bolt with a hole in the centre. The third and most crucial part is a rubber washer that fits in the centre. The bulkhead is set inside the hole so that the washer and nut are both on the outside of the tank. By setting the bulkhead this way the rubber washer is only minimally exposed to saltwater. As a result the saltwater does not dry and shrink the washer as quickly as would occur if the washer were constantly submerged. The bulkhead should fit snugly within the hole in the aquarium wall with there not being more than a 1/8-inch gap between the bulkhead and the edge of the hole. The closer the two fit the less likely there is to be any leakage. If this gap is too large it can be filled in with silicone rubber. This should be allowed to dry completely before it is exposed to any water.

Because of the relative difficulty of finding bulkheads, you may wish to buy them first, and then buy the PVC and fittings later. Right now, I have systems using 1/2" and 3/4" bulkheads. If in question, err on the size of caution by buying pipe big enough to carry more water than your pump could possibly send through the system.

Regardless whether you use clear tubing or PVC, you'll need to measure the distances between the tanks. Also, because your PVC may have to make a few twists and turns to get from one tank to the other, it's a good idea to draw a scaled blueprint of your plumbing system and, before you begin, make sure you have the right fittings. Cut the PVC to the proper lengths using a hacksaw and then, before gluing the pieces of pipe and fittings, put together the entire system to make certain the pieces properly connect your tanks together. If it works, pull the pieces apart and glue them. They should be allowed to dry for 24-48 hours to allow the glue to cure completely. Please note that after applying glue and placing the pieces together they will set in a few seconds. Make sure you glue the right pieces together in the right position.

Be careful when choosing glue. I always use aquarium sealant. Some silicone is ammonia cured (obviously a no-no in our tanks) but aquarium silicone is vinegar cured. I have heard that it's okay to use plumber's glue but I have not tried it.

Finally, if you have a UV unit, you can plumb it between the tanks to utilize the same pump or return. You just treat the UV unit like a piece of PVC pipe that the water must pass through. Incorporating a UV sterilizer may make your plumbing design slightly more complex but make your overall system look a little cleaner and less like a science project.

Connections:

Overflow connection: If you are using a hang-on overflow box, you may prefer using flexible tubing. Just slip the tube over the barbed fitting on your overflow, and secure with a hose clamp. With a drilled overflow, PVC should be glued securely into the bottom of the bulkhead.

At the tank end, you have a choice to make. Either let the tubing just hang into the tank compartment, or drill a hole in the side of the tank and install another bulkhead fitting. (Obviously, with a glass tank, you may have to just let the gravity-return line hang into the tank. ) Secure it in some way (tie straps, cord) so it can't flop out and spill water on your floor. If using PVC, you can use elbows to curve the pipe as necessary and drain as deeply into the tank as you like. My PVC goes all the way to the bottom of the main tank, ensuring some water flow in the nethermost regions of the tank.

Pump line connection: Connecting the plumbing to the pump will vary on the pump design. Many pumps have some sort of barbed fitting on top. You may have to use a fitting called an "enlarging coupling." For instance, my Rio has a 3/8" fitting, but my PVC is 3/4". I have the Rio fitting inside a piece of flexible tubing that is glued inside the PVC. I'm sure there's a better fitting to use but this is the best I've found.

III. Building the Refugium

As I stated earlier, I liked the idea of the refugium after seeing what wonderful results I had achieved with a freshwater planted tank. I had also recently purchased a few mated pairs of seahorses and I thought a refugium might make a good nursery tank for several reasons. With 14 horses in my main tank, I was slowly seeing a rise in nitrates, no matter how many water changes I did. I hoped that the macroalgaes would perform a denitrifying function. I wanted to maintain pH over the 24-hour cycle, I wanted a place where copepods could reproduce for the fry, and I liked the idea that the fry would have good water quality (both due to the presence of the macroalgaes and the larger total volume of water). I had a few other design elements in mind that would make it "fry friendly."

The Stand:

After the plumbing, deciding how to place the tank was the most difficult item to figure out. I wanted the refugium to sit over the main tank and I did not want to obstruct the view of the main tank. I definitely did not want to break down or move the main tank. I needed something I could just slip over it. The design I had in mind looked a little like those units that sit on four legs over a toilet. The problem was building something that would withstand the weight of the refugium and be stable enough not to fall over (since there wouldn't be a shelf near the bottom to brace the legs).

I looked at building such a unit but finally decided that my woodworking skills (and available tools) weren't yet at the point that I felt comfortable trying it.

I finally went into a store called Organized Living that sells modular metal units. You buy the legs and shelves in the sizes you choose and put them together to create a unique unit that serves your purposes. The 12"x36" shelves are supposed to hold up to 350 pounds, and are made of an open mesh design that was spaced enough to fit the PVC through it. I bought two shelves. I placed one just high enough above the main tank to allow the hood to open completely. The second shelf sits at the top of the unit's 74" legs, just above where the refugium would be. (The top shelf serves little purpose except to brace the unit. ) I also purchased additional braces to connect the legs at the bottom and sides for stability. I was already using one of these units for my entertainment centre so a matching shelf looked good in my living room. It was an attractive, albeit expensive, solution to my problem.

The Tank:

As with a sump, there are no set rules as to what size your refugium should be. Some places sell in-tank refugium that are probably less than a gallon in volume; you could conceivably have a refugium that was ten times the volume of your main tank. The things to keep in mind are the weight capacity of the stand you are using, how much pump you can afford and the amount you want to spend on lighting-remember, the larger the tank, the more light you will require.

My first attempt at a refugium used a 20-gallon glass tank. I couldn't find anyone to drill it for a reasonable price (I was quoted a price of $40 per hole with no guarantee against breakage). I first made a DIY overflow box. I know that these boxes can work fine but I wasn't comfortable with it, largely because I recognized the likelihood that fry would get sucked into it. It also required too much clearance in the back of the tank and just didn't look neat. Possible overflow due to loss of siphon was of particularly concern in a refugium full of macroalgaes — one stray piece in the overflow box could wreak havoc.

I finally decided to change the refugium to an old ten-gallon acrylic tank I had and that I was able to drill myself.

I purchased two 3/4" bulkheads from my local LFS. I wanted the bulkheads on the bottom of the refugium, on either end. I'll describe more about the design later but, basically, one hole was for water to flow in and the other one was for the overflow down to the main tank.

I didn't have a routing attachment on my drill so, instead, I drew a 7/8" circle on the acrylic and drilled a bunch of tiny holes along the line. I then just punched out the hole where the bulkhead would be fitted and glued.

My overflow. Most overflow chambers utilize a design whereby the water flows from the top of the main water chamber into the overflow chamber, from where it drains down to the main tank. In this way, tiny critters in the main chamber of the refugium would eventually make it down to the main tank. Because I hoped to use my tank as a rearing tank, I wanted to avoid the possibility of tiny fry getting sucked into the overflow.

My overflow chamber makes up about 1/4 the volume of the refugium. I built the chamber by cutting two pieces of Plexiglas to make the chamber walls. Each piece is the width of the refugium tank, and about two-thirds the height. Using silicone sealant, I attached one piece from the top of the aquarium going down, about 4" from the end wall (leaving 3.5" clearance on the bottom). I placed the second piece of Plexiglas parallel to the first wall and about an inch from it (away from the tank wall), but from the bottom up (clearance at the top). Between the two pieces of Plexiglas, I placed a sponge to prevent the fry and critters from flowing through to the overflow chamber.

(See diagram. )

The overflow's standpipe is a piece of PVC that sits in the middle of the overflow chamber and runs from the bulkhead up to the height of the refugium (11"). I drilled a bunch of holes in the top of the standpipe to just below where I expected the minimum water level to be. This way, if the pump shuts down, water will never drain from below the level of the drilled holes in the standpipe.

(See diagram)

Water In:

Because I knew that one of the challenges of raising reidi and/or OR fry is to prevent them from sticking to the surface of the water and sides of the tank, I wanted to incorporate a surface spray bar into the water pump line. I did this by sizing PVC and connections to fit inside the perimeter of the main refugium chamber (approximately 14"X 10"). The four pieces of PVC are connected using 90-degree elbows in the corners. To make it work like a spray bar, I drilled 1/8" holes approximately one inch apart along the entire length of the PVC. When I put the pieces together, these holes were situated to spray out against the tank walls, creating surface turbulence (to prevent fry from sticking and also for air/gas exchange). I then placed a solid piece of PVC at the end of the tank, upright from the bulkhead to the top of the tank wall, and attached my spraybar using a "T" joint. Water comes up through the bulkhead, rises up the PVC and, on reaching the top of the tank, is disseminated around the entire inside of the tank walls at the water surface by means of the spray bar.

(See diagram. )

Connections and the UV Unit:

Time for plumbing! I diagrammed the two systems to figure out where my plumbing lines should be. There were a couple of things I had to take into consideration: the size and shape of my existing hood and the size of the pump I wanted to use. I have an Eclipse 3 hood on my main tank and I wanted to be able to use the existing openings on either end. These openings were, however, farther apart than the length of the 10-gallon and I didn't want to cut any extra holes into the Eclipse. This meant that I would have to include some elbows in the design, as well as both vertical and horizontal PVC (because the water couldn't run straight up to and straight down from the refugium into the existing openings in the Eclipse hood). As for the second consideration, I had an extra Rio 800 but the flow rate was a little higher than I wanted through the refugium. I decided to include the elbows in the line up (remember, increased head decreases water flow) and then have the gravity return perfectly vertical.

The pump sits in the far back left corner near the surface of the main tank. I connected it using a fitting called an enlarging coupler (the fitting in the Rio is 1/2" and my PVC was 3/4"). I used a 90 degree elbow to go up the tank and through the back of it, a 1" piece of PVC and another 90 degree elbow to reach the refugium (approximately 6") and another 90" elbow to reach up to the bulkhead. With approximately 5 feet of head, the Rio is rated at 100 GPH (at 0 head, its rating is 211 GPH). The plumbing looks something like this:

(See diagram. )

The overflow design was much simpler. Initially the PVC went straight down from the bulkhead to the Eclipse hood opening, a 90 degree elbow into the opening and another 90 degree elbow to go down into the tank. The overflow drains at the bottom of the main tank where clean, fresh water and good circulation are most beneficial.

(See diagram)

I later purchased a vertical UV unit and incorporated this into the overflow. Water now flows into the UV sterilizer, through it, and then out into the main tank. Because of the design of the unit, I discarded the PVC in favour of clear plastic tubing.

Lighting:

I wanted the top of the refugium open for air/gas exchange so I didn't want a hood. I knew that the macroalgaes I hoped to grow in the tank would prefer light in the red spectrum of the average horticulture grow light (most corals and coralline algae thrive under a more specialized blue light spectrum). Finally, because of my desire to make it "fry friendly," I also didn't want the lights at the top of the tank.

Lighting turned out to be my least challenge. I purchased a couple of inexpensive clamp-on fixtures and installed 60-watt grow bulbs in them. I attached the fixtures to the shelf beneath the refugium so that light would be provided from the bottom, not the top, of the tank.

I also took wide black electrical tape and wrapped it around the top of the tank to darken it, and to hide the spray-bar. I will probably eventually devise a way to cover the top of the tank to darken the surface and thus discourage the fry from swimming toward it.

Substrate:

Because I already had a deep (denitrifying) live sand bed in my main tank, I didn't need much depth in the refugium and, with only about 10" to work with below the spray bar, there wasn't much room.

I had purchased several 40-pound bags of Home Depot Playsand for about $5 a bag. This sand is a fine grade aragonite and just about exactly the same stuff that aquatic stores package differently and sell for $50 a bag. If you can find this stuff, it's great for aquariums. Just make sure you buy the Caribsea variety and not the regular Home Depot sand, which contains silicates. I was fortunate to purchase my sand from a local reefer who imported a pallet for his tank. I believe that the Caribsea sand is only available at Home Depots on the East Coast.

I used about half a bag of the sand to create a depth of 1.5 inches.

Critters and Macroalgaes:

The sand bed in my main tank was full of very diverse life. Over a year earlier, I had purchased a Sand Activator kit from Indo-Pacific Sea Farms, and had subsequently ordered GARF grunge and gotten additional sand from several local LFSs and area reefers. I moved about a cup of that sand to the refugium.

From Inland Aquatics, I purchased the refugium flora and refugium fauna kits. I was very pleased with the quality and quantity of the macroalgaes in the flora kits but I was very displeased with the fauna kit. IA was out of copepod cultures when I ordered, and it didn't seem like they made any substitutions. It was a lot less stuff than I'd gotten from IPSF.

I also purchased a ball of sea moss, some sea grass and the green amphipods from Ocean Rider. If you are placing an order with OR and wish to start a refugium, Carol offers a nice variety of macroalgaes you can add and will help you choose species that will work within your available lighting. The amphipods have proven to be one of the most prolific breeders in that tank, and they are just adorable.

A word of caution, however. Whatever you buy and wherever you buy it, check carefully for unwanted critters. Although I can't with certainty identify the sources, shortly after adding the macroalgaes, my refugium broke out with a disastrous case of aiptasia, anemonia, and hydroids. I think I was able to eradicate most of the hydroids (I only saw 2 or 3) but I'm still battling aiptasia. The peppermint shrimps I added recently seem to be doing a good job of taking care of it though.

IV. Total cost

The following are merely estimates-for items that I already had, I included a reasonable value if you needed to purchase them. (I probably don't really want to know this. )

V. Was It Worth It?

The answer is a resounding YES!

I can't even explain the feeling of satisfaction I got when I first plugged in the Rio, heard the water rise and then start streaming out of the spray bar, fill the overflow and then effortlessly flow back to the main tank. I checked carefully for leaks and found none. The sense of accomplishment in building something that worked so beautifully was a reward in and of itself.

I've had mixed results with the macroalgaes. Some thrive in this refugium (feather, grape caulerpa) while others, particularly the sea grasses, have just turned brown and withered away. Nothing has grown tremendously, as I had hoped. Nonetheless, there has been no measurable nitrate since I added the refugium, and I've noticed an obvious decrease in the amount of problem algae in the main tank, since undesirable algae now has to compete with the macro's in the refugium.

The growth of critters has been fascinating. I used to really enjoy watching the 'pods who, for obvious reasons, disappeared as soon as I added horses to my tank. The refugium has a plethora of 'pods and other critters.

The reverse photocycle has completely stabilized pH. I run the main tank from 8 a.m. to 8 p.m. , and the refugium from 8 p.m. to 8 a.m. My pH is at 8.5, whether I measure in the morning or evening.

There are two benefits that I hadn't considered. First, the overflow chamber makes a great feeder. I've put a bunch of red feeder shrimp and brine in it when I was leaving for a few days and most of them eventually get caught in the overflow and end up in the main tank. When I returned from a vacation, I noticed my horses all hanging around the exit of the overflow, waiting for more red shrimp. It happens slowly enough that there is nearly a constant supply of food.

The other benefit is that the refugium makes a nice infirmary. When want to isolate a horse for feeding or recovery, the refugium is a good place to keep an eye on him, let him feast on the critters for a few days, or feed him special foods that I don't want the others to get. Putting him in the refugium, which shares a water source with the main tank, avoids the shock of moving into water with a different temperature, pH or chemical makeup.

As for using the tank as a nursery; again, mixed results. Two of my barbouris were in the refugium and the male unexpectedly gave birth to a small brood on Dec. 31. Unfortunately, I didn't notice the babies until late in the day, and most had been killed by aiptasia. I immediately added three peppermint shrimp that took care of a good deal of the aiptasia within a week.

Again on January 12, my barbouri had a brood, this time consisting of over 100 babies that I noticed immediately in the morning. I didn't have time to do anything with them and, as I had another vacation planned, wasn't sure if I wanted to put the time into rearing them when I knew I'd be gone for 10 days at their 4-week point. (I was also concerned that they might be premature as this was such a short gestation period and they seemed much smaller than the brood born 12 days earlier. ) I noticed that they were having trouble hitching but the spray bar did its job-the force of the spray bar is pushing down the babies trying to reach the surface. Some of the babies have survived feeding on the copepods and rotifers in the refugium, without any additional feeding. It's hard to say how many there are, as they are hiding among the macroalgaes.

VII. What I'd Do Differently

The only thing that has really displeased me about the design is the placement of the bulkheads. Because they're on the bottom of the tank and because of the design of the stand, the refugium tends to tilt to one side or the other, causing the spray bar on the higher side to be more of a drip bar. If I were to do it again, I'd plumb it from the back instead of the bottom. That's really the only change I'd make.

VI. Future Additions

Because of the plumbing, I had to remove my protein skimmer from my main tank, and I haven't yet figured out how to incorporate one into my present system. Right now, it doesn't seem to be a vital piece of equipment but I may add one in the future.

I'm not completely pleased with the lighting and may at some point upgrade it to allow for better growth of sea grasses. I'd also like to add a mangrove.