What It Is, Its Applications for Seahorse Rearing, and How To Culture It In 6 Easy Steps

Will Wooten

Itís green (usually), itís tasty to corals, and itís essential to the food chain. Itís some of the smallest eukaryotic life, itís usually one-celled, and itís everywhere in the ocean. It comes in many forms- diatoms, dinoflagellates, pelagic microalgae, and even coccolithophoresÖ So, what is it?! Itís phytoplankton, and it can make raising those little seahorse fry much, much easier!!!

So, you ask, what does it do? Phytoplankton, when used in seahorse fry rearing systems, has a number of benefits. First and foremost, it provides a source of enrichment for their zooplanktonic food- brine shrimp napauli, rotifers, and the like. These filter-feeding microcrustaceans, when released into the seahorse rearing system, immediately begin filtering the water for any particulate matter they can find before being sucked into the snout of some hungry fry. But any time counts, and the more nutritious phytoplankton your seahorsesí food can consume, the more nutritious phytoplankton your baby seahorses will consume.

But donít change the channel; thereís more! Being miniature Ďplantsí (ok, theyíre actually protists, but thatís scientific mumbo-jumbo), phytoplankton are able to utilize nitrogenous compounds, i.e. the ammonia in baby seahorse poop, as a source of fertilizer. So, essentially the phytoplankton serves as a means of biological filtration, thus keeping your rearing systemís water more stable and cutting down on the waste levels in the water (but yes, you will still need to do those always-essential water changes almost every day).

And it doesnít end there, friends. There are many small benefits to using phytoplankton in the seahorse rearing system. It provides a screen against light that can disorient or stress your fry. Additionally, as marine fish are constantly consuming the water in which they live, the fry will essentially be living in a vitamin smoothie.

So, now you know what phytoplankton does, but where will you get it? Itís available cultured from many online dealers, and there are a few commercial companies that offer live phytoplankton through local fish stores. The most practical and cost-effective means of getting phytoplankton, however, is by culturing it yourself. Itís really quite simple.

To start out, you'll need a few pieces of equipment that should be relatively easy to come by:

  • a 2-gallon hex aquarium starter kit; these commonly-available kits come with a tank, a light socket, an air pump, and various other items. They are available almost anywhere, including Wal-Mart and other fine value stores.
  • a 7w fluorescent light bulb; the type that screws into a standard incandescent outlet. Make sure you do not get higher than seven watts, as anything more powerful can overheat and crash your algae culture.
  • Miracle-Groģ Water Soluble All Purpose Plant Food; i.e., the blue stuff. I've found that no other fertilizer works quite as well as this stuff. It's water soluble, easy to find, and its blue colour serves as an indicator of when you should add more (as the blue fades, add more).
  • approximately 1 gallon natural seawater; water from subtropical or temperate regions works best because it contains higher initial levels of phytoplankton. The best time to collect is summer, as this is when levels are highest. several gallons new artificial seawater; you will need to use new artificial seawater as opposed to old tank water because tank water can contain zooplankters that will feast on your algae cultures, reproduce like mad, and cause the culture to crash. THIS IS VERY IMPORTANT!

Once you've found each of the above items, follow these directions:

Step 1. Remove the tank from its box so that you have only an empty acrylic aquarium. Remove everything inside of the aquarium such as the air pump, the under gravel filter (which can be thrown away as you will not need it for your culture), etc. Pour the gallon of natural seawater into the aquarium. It should fill it slightly more than halfway.

Step 2. Fill the aquarium to the top with the remaining new artificial seawater. Then add the air stone/airline tubing. Plug in the air pump to make sure it's bubbling properly. The air stone must stay on constantly — 24/7.

Step 3. Remove the 7w fluorescent light bulb from its packaging and screw it into the hood of the aquarium. Plug it in and turn it on to be sure that it works properly. The light will stay on constantly, even at night. Obviously, the setup pictured has been in use for quite some time. ;)

Step 4. Add approximately 1 tsp. Miracle-Groģ fertilizer to the aquarium and dissolve it thoroughly. Then fix the aquarium cover and light hood onto it so that a tight seal is made.

Step 5. Wait ... Continue adding 1tsp Miracle-Groģ every other day. In about a week or two (depending on initial phytoplankton levels in the water you've collected), the aquarium water should be a deep, emerald green. At this point you will no longer need to add fertilizer. When the water has reached this colour, remove half of it and replace with new artificial seawater. Do not add any extra fertilizer. In anywhere from three days to a week, the water will have returned to the deep, emerald green colour. When it has, again remove half of the water and replace with new artificial seawater. Once the water has again become deep, emerald green, it is ready to be used.

Step 6. This is the fun part! Now that you have cultured your very own phytoplankton, put it to good use! Encourage your seahorses to have fry, and raise them large and healthy with your magical planktonic soup! You can also use it to feed invertebrates in your reef tank such as clams and feather duster worms.

Well, there ya have it. I told you it wouldn't be too hard! If the case arises where your culture doesn't work out, however, there are several other things to try:

  • Instead of starting from scratch, use a starter culture from a friend. If you do this, skip steps 4 and 5. The culture should grow on its own to reach that deep, emerald colour.
  • Use another source of live phytoplankton such as DT's as a starter culture.
  • Purchase an "algae disk", available from several online retailers. These disks contain freeze-dried phytoplankton cultures and are easy to use per the manufacturer's instructions.

If still you have trouble getting your phytoplankton culture running, feel free to e-mail the author with questions. Good luck, have fun, and happy seahorse keeping!


Gordon, Arnold L. 1994. Ocean. The World Book Encyclopaedia Volume 14: p. 655, Chicago, IL.

Most recent revision: 20 January 2004

Copyright © 2004
Will Wooten &
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