Why Did My Fish Die? The Common Causes
Most sudden deaths trace to water, not bad luck.
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Water quality first
The #1 killer is invisible: ammonia or nitrite from an uncycled or overstocked tank. Test your water — if ammonia or nitrite is above zero, that's almost certainly the cause.
Temperature and oxygen
A heater failure, a cold water change, or a hot room can shock fish. Poor surface agitation can also starve a tank of oxygen overnight. Check your heater and add surface movement.
Incompatibility and stress
Aggressive tankmates, the wrong water parameters, or too small a tank cause chronic stress that kills slowly. Match species carefully — that's what our compatibility tools are for.
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FAQ
Why do my fish keep dying?
Test your water first — ongoing deaths almost always mean an ammonia/nitrite problem or wildly wrong parameters, not disease or chance.