Why are my fish dying when water parameters are OK?

First: rule out the basics

When fish die and ammonia and nitrite read zero, it is easy to assume water quality is fine. But "OK" can mean different things. Check: Are you testing with fresh reagents? Is the test kit in date? Did you recently do a large water change that temporarily diluted ammonia? Could there have been a spike between tests? See water parameters and suddenly losing fish for immediate steps.

Disease and parasites

Internal parasites, bacterial infections, or viruses can kill fish without obvious outward signs. Fish may stop eating, hide, or die suddenly. Common illnesses like ich or neon tetra disease can spread. If you added new fish without quarantine, disease could have been introduced. Use a hospital tank to isolate and treat sick fish.

Aggression and stress

Bullying or territorial fish can stress others to death. Chasing, nipping, or blocking access to food and hiding spots causes chronic stress. Check compatibility and watch for aggression. Add more hiding spots or rehome the aggressor. See are guppies aggressive for an example of how stocking ratios matter.

Chlorine and chloramine

Untreated tap water contains chlorine or chloramine, which burns gills and kills fish. Always use a dechlorinator before adding water. If you forgot once, that could explain sudden deaths.

Oxygen

Low oxygen can kill fish, especially in warm weather or heavily stocked tanks. Ensure good surface agitation (filter outflow, air stone) and avoid overloading the tank. See fish gasping at surface for signs and fixes.

Temperature shock

Sudden temperature changes — from a water change, heater failure, or adding fish without temperature matching — can shock or kill fish. Match new water temperature to the tank and acclimate new fish properly.

High nitrate

Ammonia and nitrite are the usual suspects, but very high nitrate (e.g. over 80–100 ppm) can stress or harm some fish. Regular water changes keep nitrate in check.

Old age or hidden illness

Sometimes fish die from age or conditions that were not visible. If one fish dies and the rest are fine, it may be an isolated case. If deaths continue, keep investigating.

What to do now

Remove dead fish immediately. Do a partial water change. Test again and log results. Observe remaining fish for symptoms. If more show signs of illness, isolate and treat. If aggression is likely, adjust stocking or add cover. Track parameters with App-aquatic to spot patterns over time.

Quick takeaways

• Disease, aggression, chlorine, oxygen, and temperature shock can kill even when ammonia and nitrite are zero.
• Quarantine new fish; use dechlorinator; match temperature on water changes.
• Observe, isolate sick fish, and keep testing.