Seawater Quality

As has been often stated the number one quality requirement in the marine aquarium is seawater quality. This is logical to everyone (or maybe nearly everyone!). The number two, at least in the reef aquarium, is lighting. So let’s have a look at seawater quality.

Obviously seawater is in direct contact with fish and corals and is the environment for them. Aquarists have no problems in checking seawater quality as there are plenty of test kits available for purchase. The test kits are not ‘scientific’ as they measure less accurately than scientific versions, but the kits are fine for the aquarist’s purpose.

There are three major areas for checking, the first is ammonia, then nitrite followed by nitrate. Ammonia (NH3) is very toxic to aquarium creatures and ‘burns’ their flesh. Therefore ammonia should be near or best at zero on the test kit. Nitrite (NO2) is the intermediate product and is also toxic but not as bad as ammonia, and should be undetectable on the test kit. Nitrate (NO3) is the end product of the cycle and is not as harmful in low amounts but can cause frustration: unwanted algae could appear, the algae uses the nitrate as food. Therefore fish should not be overstocked or overfed as leftovers could rot down and eventually cause the mentioned algae which is not beautiful in a display aquarium. Nitrate tests should show levels lower than 10ppm (parts per million) or undetectable.

There are other important measurements that are required and again test kits are easily available. The first of these is pH which shows seawater acidity or alkalinity. The scale used is 0 to 14, zero is acidic, so 7 is neutral. A reading of 8.2 to 8.4 is fine for captive seawater. There could be variations during the day with bright lighting driving photosynthesis. Therefore if the aquarist finds that the reading seems high – are the lights on? Take another reading before the lights come on and compare. pH that is a little higher than 8.2/8.4 could have the advantage that algae doesn’t like it. However, a ‘normal’ reading is best.

Another test that is advantageous is alkalinity. Natural seawater measures around 2.5meq/L. An aquarium level of 3.2 to 4.5meq/L can be advantageous. Some test kits for alkalinity measure in dKH, a range of 7 to 12 dKH is recommended.

If a test results in a measurement that is way off then checks (on line for example, or in a modern book) should be made to find out what this could mean. Often the way forward is a percentage seawater change, the percentage being low or high depending on the numbers resulting from the test. The start percentage is usually 10% of the aquarium gross gallonage for standard maintenance, being increased if necessary.

It is important to do a standard partial seawater change on a fixed basis to help maintain seawater quality, the percentage for this is normally 10% of the gross gallonage. Reasonable attention to seawater quality helps keep away problems.


  1. Good good information on your main page, from the first read

    I’ve had a saltwater aquarium for about 10 years now or actually more than 10 years more since a house reno. Moreover good stuff and wish I would have read this years ago

    Patience and perseverance is key

  2. You’re quite correct, patience and perseverance. Obtain the information, then apply it. As time passes and experience grows procedures become easier and second nature. You have experience which is so valuable. Thanks for calling and hope you do so again.

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