Mastering Freshwater Aquarium Ecosystems
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Water Testing (Cont.)

In an established aquarium you should not have to test for ammonia unless your fish look stressed, or you have unexplained deaths. Some things that have been known to cause elevated TAN readings in an established aquarium are:

  • Power outage for an extended period
  • Gravel cleaning with a gravel vacuum on a tank that does not have an undergravel filter
  • Cleaning a sponge filter
  • Cleaning a canister filter
  • Low aquarium pH
  • Antibiotics

The toxicity of NH3 is highly variable depending on the species of fish.  Damage to fish organs can occur with exposure to low levels of NH3 for an extended period of time. The below table provides some conservative general guidelines to follow for determining how toxic the NH3level is in the aquarium:

NH3 ppm Affect on Fish
0.020 - 0.049 Can cause long term harm that will affect growth, immune system, and health, especially to eggs and fry
0.050 - 0.199 Can be tolerated for only a few days and is harmful
0.200 - 0.499 Can be tolerated for a 24 to 48 hours and may kill some species of fish
>0.500 Deadly to some species of fish within 24 hours of exposure


Nitrite levels in a new aquarium will normally start to rise about two weeks after it has been set up with fish. At two weeks you should start testing nitrite levels at least once every two days. Nitrite levels will usually peak at about three and a half weeks after adding fish to a new aquarium. Nitrite levels should be down to near 0 ppm at around 35 days. If toxic (=> 2 ppm) or near toxic nitrite levels are detected, a water change will be required to reduce the concentration. Once the nitrite concentration in a new aquarium starts to drop without doing any water changes, you can then test every five days until it drops to 0 ppm. Once the ammonia and nitrite levels stay at 0 ppm the aquarium is then considered cycled.


Nitrate in most freshwater aquariums will accumulate over time. How fast it accumulates will depend on stocking densities, feeding, live plants, and light intensity and duration.

In a low stocked aquarium with live plants and high intensity lighting, it is possible to have no noticeable nitrate build up. An aquarium that does not have any nitrate build up is considered balanced and should be what all aquarist strive to achieve.

You should start testing nitrate level after a new aquarium has been set up for two months. If you have any nitrate build up, this should give you an initial idea of how fast nitrate will build up with the current bio-load. You should continue to test the nitrate several times a year to make sure you are keeping nitrate under control.

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