Testing Water Quality

By Richard Webber

What you test for, why you test for it, and what the acceptable values are.






Ammonia (NH3NH4+)



Ammonia is the primary enemy of invertebrates & fish, capable of causing death in very low concentrations. Causes include: Immature filter, overfeeding, overstocking & dead stock.

Nitrite (N02)



Even trace levels of nitrite can destroy a well presented invertebrate aquarium & cause fish much distress.

Nitrate (N03)

below 15 ppm


A good overall indicator of general water quality & one that should be kept extremely low if invertebrates are to thrive. Constantly high nitrate levels usually reflect high fish stocking ratios. 


1.021 - 1.024


Salinity measures the total amount of dissolved solids in sea water. It is usually recorded as specific gravity ( S.G.) But can also be referred to as parts per thousand (ppt). Constant evaporation of freshwater from the aquarium causes the salts to become more concentrated & the salinity to rise. To maintain stability regular addition of freshwater is needed.


8.1 - 8.3

initially daily, then weekly

pH is a measure of the alkalinity or acidity of aquarium water. Some natural changes are to be expected during the day. Aquarium water could drop to as low as 7.9 at the end of the night, & peak at around 8.4 just before lights out. These natural pH cycles are gradual & tend not to stress livestock. 

Phosphates (PO4)



Invertebrates do not prosper when levels of phosphate get high. Phosphates arrive in the aquarium through unfiltered water, poor quality carbon & marine salts, but mostly through fish waste products. Nuisance algae will thrive where phosphate levels are high. High-quality water changes or phosphate removing resins can help alleviate the problem.

Dissolved Oxygen (02)

6 - 7 ppm

Monthly, or on demand

Both fish & invertebrates benefit from high levels of dissolved oxygen. Good water circulation is the key, as oxygen is drawn mainly from air & water. Dissolved oxygen also affects pH.



Intermittently, then on demand

Copper-based medications have proved very reliable in the treatment of various fish diseases. It is highly toxic to invertebrates & should never be used in aquarium housing these animals. Copper adversely affects seahorses. Copper can be introduced to the marine aquarium by way of domestic water & this should be tested from time to time.


350 - 400 ppm


Calcium is a vital element in the marine aquarium. A host of invertebrates draw it from the surrounding water and calcium reserves need to be replenished on a regular basis. Regular water changes usually achieve this. A reef tank may require the addition of biologically available calcium to keep levels optimum.

Carbonate Hardness (KH)



KH is a measurement of various carbonates & bicarbonates of calcium & magnesium, & borates in sea water. A stable KH will prevent rapid declines in alkalinity & subsequent drops in pH. Boosting the KH of aquarium sea water to between 12 - 18dKH using a proprietary generator has been recommended. However, left to their own devices, most aquariums settle naturally to around 7dKH & there appears to be no advantage in constantly increasing dKH to unnatural levels.

Most recent revision: January 23, 2004

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Richard Webber & Seahorse.org
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