Resembles oxygen deficiency externally, which is recognised by heavy breathing, rapid gill movement and hanging around the water surface. Rubbing is also a frequent symptom. The toxicity of nitrite (NO₂) (which can be checked with a test) is the reason and causes internal asphyxiation of the fish because it blocks the transport of oxygen in the blood. Nitrite levels of 0.2 mg/l and more (with more sensitive species from oxygen-rich, fast-flowing bodies of water 0.1 mg/l and more) are already dangerous and need remedying. The causes of elevated nitrite levels are: disturbed bacterial balance in the tank as a result of bactericidal drugs, very low or high pH levels (< 6 or > 8), a too large fish population, too little (biological) filter output or too much food. In saltwater, the salt reduces the toxicity of nitrite. This is also why salt is often added to the water when freshwater fish are being transported or kept in intermediate storage. Remedy: Depending on the level of the nitrite level, 30 - 50 % partial water change. If the filter is cleaned thoroughly or the filter media are replaced, do not do this at the same time as a water change.