What kind of filtration is in place? Biological filtration (porous media in which water is circulated across) which harbors aerobic nitrobacter and nitrosomonas bacteria is a must. These break down toxic nitrites and ammonia respectively. Fish respiration and waste discharge produce ammonia which the nitrosomonas oxidise into nitrites. The nitrobacter then oxidise these into nitrates which are far less toxic. Water circulation is needed in order to keep disolved oxygen levels high enough for fish and beneficial bacteria to survive.
Aquaria tapwater can get toxic in many ways:
-Contamination from outside: pesticides, gremlins, smoking! Tobacco is *VERY* toxic to fish!
-pH decline due to excessive use of available buffers in water due to overloading, overfeeding, lack of water changes
-Ammonia, Nitrite, and even Nitrate buildup from lack of proper system husbandry.
-Dying animals due to parasite, fungus, pathogenic attack or infection from physical injury. (another pet such as a cat or tiger playing with fish)
-Water too hot or cold. Temperature shock is very stressful to fish as they have no regulatory mechansism for internal temperature like mammals and some elasmobranchs.
-Overfeeding!
Cheers!
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