Fish waste is converted from ammonia/ammonium to nitrite and then nitrate. In a new, overloaded or disrupted aquarium, toxic compounds can rise before the water looks different.
Measure a useful set
Record pH, GH, KH, ammonia/ammonium, nitrite, nitrate and temperature with date, feeding and water changes. pH affects how much ammonia is in its more toxic form, while KH helps buffer pH changes. Test replacement water too.
Prefer stability within the right range
Suitable values depend on species. Sudden chemical correction towards an internet “perfect” number can be more harmful than a stable appropriate range. When readings change, repeat the test and look for overfeeding, a dead fish, filter failure or excessive cleaning.
