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Mastering Freshwater Aquarium Ecosystems
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Nitrogen Cycle (Cont.)

If you are trying to keep Tanganyikan cichlids in water with a pH of 9.0, that has a TAN of 5 ppm your ammonia level is 2.06 ppm (a deadly danger zone). This is why saltwater fish and African cichlids are thought to be more sensitive to ammonia, these fish are normally maintained in water with a pH of 8.2 or greater. At a pH of 6.0, and 10 ppm of TAN, the ammonia is only .007 ppm. While it looks like the fish mortality should be very high, the fish are doing fine. The graph below provides a "True Free Ammonia" chart that can be referenced for figuring out how dangerous your TAN reading is.

Free Ammonia Chart

The "Free Ammonia" chart above shows the amount of free (toxic) ammonia (NH3) in the TAN reading at specific pH reading. Colored lines indicate the reading from the aquarium test kit which is NH3 and NH4+. Free ammonia measurements on the left of the chart indicate ppm. Notice that above the pH of 8.0 the toxicity of the TAN rapidly rises. You must test the pH of the aquarium to find out how much free ammonia is in the TAN reading.

South American and West African cichlid breeders that maintain a low pH below 6.0 need to be cautious when performing water changes, as the low pH has an adverse affect on the nitrifying bacteria that converts ammonia to nitrite. Because of the acidity, these bacteria populations can drop so low that the TAN reading can rise quickly. While the pH stays low the TAN reading is nearly all ammonium, but if you do a water change or add a alkalinity buffer to the system the ammonium can be quickly converted to ammonia, potentially causing ammonia poisoning.

Free Ammonia Part of TAN

Temperature Conversion


Enter Ammonia test kit TAN ppm (NH3 and NH4+) result
Enter pH test kit result
Enter Temperature in °C

Amount of NH3 (Free Toxic Ammonia in the TAN)

If you know your know your temperature in Celsius skip "Temperature Conversion" fields. Enter your ammonia test results in the first field, then enter the pH from test results in the second field, then enter the temperature in °C in the third field. Click on "Calculate" button to see the amount of toxic ammonia in the TAN. A NH3 level of .6 or greater can kill many species of fish within 24 hours of exposure.

Ammonia levels will normally peak around 2 weeks after adding the first fish. In an established aquarium the ammonia level should always read 0 ppm. Often in freshwater aquariums, fish will survive the ammonia spike when a new tank is cycling, because the pH is low enough that the concentration of true ammonia stays low. It is the nitrite that is more often the killer of fish in a new aquarium setup.

Nitrite (NO2)

In an established aquarium nitrite level should always be at 0 ppm. Nitrospira bacteria oxidize nitrite into nitrate. Nitrite is extremely toxic to fish. It only takes .5 ppm of nitrite to cause stress, and 2 ppm to kill fish. Nitrite in the water will displace oxygen in the fish's blood, causing a condition called methemoglobinemia or “brown blood disease.” Nitrite is primarily absorbed through the gills and digestive tract and attaches to hemoglobin. This prevents the uptake of oxygen.

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