The Essential Water Tests for Saltwater Aquariums

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A saltwater aquarium is a closed system.  The water just goes around and around, and the only things that go in and out are the things that you, as the lord of your domain, add or remove.  Water chemistry is key to maintaining a healthy FOWLR or reef tank environment.  With that in mind, here are the essential water tests for saltwater aquariums.

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Ammonia is likely the chemical you will have the most difficulty maintaining in a new saltwater reef fish tank.  Until your rock has a chance to build up beneficial bacteria that eats ammonia and turns it into nitrite (known as the nitrogen cycle), it will be imperative that you carefully monitor your ammonia level.

Ammonia is a byproduct of the fish in your aquarium.  It is expelled from your fishes gills, and also occurs when some fish and food byproducts break down in the water. If these levels get too high, your fish will likely become sick and die.

Ideal Ammonia Level: 0
Warning Levels: greater than .25


The second part of the nitrogen cycle, nitrite is usually produced by beneficial bacteria breaking down and eating ammonia.  This is also a sign that your tank has begun cycling, when you are first setting up your tank. 

While not as immediately harmful to fish and corals, it will make fish and other animals sick if maintained above trace amounts for any extended period of time.

Ideal Nitrite Level:  0
Warning Levels: greater than .2


The last part of the nitrogen cycle (or second to last, if you consider nitrogen gas leaving the tank), Nitrate, is the least dangerous waterborne chemical in the nitrogen cycle.  Nitrate is produced by beneficial bacteria eating nitrite.  Most saltwater fish tanks with any level of fish or other life will have some level of nitrate.  

Many algae, and some corals, will consume nitrate.  In fact, one of the easiest ways to maintain safe nitrate levels, beyond simple water changes, is to grow nitrate eating algae or plants.  We have a few saltwater and freshwater options in this article.  

However, most of the time you will just need to change your aquarium water when your nitrate levels get close to leaving safe levels, if not changing it before.  Routine water changes that are not predicated on unsafe chemical levels are the key to a successful saltwater aquarium.  

Ideal Nitrate level: 0-5 ppm
Warning Levels: greater than 10 ppm 


More of a problem for reef tanks than fish only saltwater tanks, phosphates are an interesting edge case when it comes to chemical.  Many reef tanks will have corals that need some level of phosphates to survive, but if you get too high of a level of phosphate, you may get an algae bloom.

Ideal Phosphate Level (Reef Tank): between 0 to .2 ppm
Ideal Phosphate Level (FOWLR): 0
Warning Levels: greater than .2 ppm

Other chemicals

While a bit beyond the scope of this article discussing essential water tests for saltwater aquariums, here are some other levels you should maintain.  We've created a "Pocket Guide" for correct chemical levels that can be saved for future reference:

Saltwater Water Chemistry Parameter Chart

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How do I Check my Water Chemistry?

The easiest way to check your water chemistry is with a test kit.  While at some point you may want to invest in the more expensive electronic testing devices such as those made by Hanna Instruments, we recommend starting with a Red Sea Aquarium Test Kit, that has everything you need to get started, which includes testing for ammonia, nitrite, nitrate, pH, and alkalinity. 

Now you know the essential water tests for saltwater aquariums!  Looking for a good place to ask questions about water chemestry and other topics?  We recommend checking out Reef2Reef, a fantastic forum with fantastic people who love to help out new saltwater reef tank (and FOWLR) hobbyists.