Losing fish is one of the most disheartening experiences in the hobby. Before you can fix the problem, you need to identify the cause, and fish deaths are almost always caused by one of a small number of issues. Work through this guide systematically to find what's killing your fish.
Step 1: Test Your Water Right Now
Before anything else, test your water with a liquid test kit (not strips). Test for ammonia, nitrite, nitrate, and pH. These four numbers will tell you the most likely cause immediately.
- Ammonia above 0.5ppm: this is killing your fish. See Ammonia Poisoning below
- Nitrite above 0: tank is not cycled or cycle has crashed. Urgent
- Nitrate above 80ppm: chronic poisoning from insufficient water changes
- pH below 6.0 or above 8.5: extreme pH is lethal. Test your tap water too
Ammonia Poisoning: the Number One Killer
Ammonia is produced by fish waste, uneaten food, and decomposing plant matter. In a cycled tank, bacteria convert it to nitrite then nitrate. If your cycle isn't established, or has crashed, ammonia builds to lethal levels fast.
Symptoms: fish gasping at the surface, red or inflamed gills, lethargy, clamped fins, erratic swimming.
Emergency treatment:
- Do an immediate 30–50% water change
- Dose with a detoxifying dechlorinator (Seachem Prime) which temporarily binds ammonia
- Reduce feeding to zero for 48 hours
- Add bottled beneficial bacteria
- Retest every 6 hours and repeat water changes if ammonia rises above 0.5ppm
New Tank Syndrome
If your tank is less than 6 weeks old, deaths are almost certainly due to a tank that hasn't completed the nitrogen cycle. The solution is the same as ammonia poisoning: daily water changes, no feeding, and patience while your bacteria colony establishes.
Temperature Issues
Check your thermometer. A faulty heater can cause the tank to overheat (above 30°C will kill most tropical fish within hours) or stay too cold. Never trust your heater without a separate thermometer; heater thermostats fail without warning.
Disease
White Spot / Ich: small white spots like salt grains; raise temperature to 28°C and treat with Ich medication for 10–14 days. Velvet: fine gold/rust dust; highly contagious, treat the whole tank with copper-based medication. Fin Rot: ragged, disintegrating fins; fix water quality first, then treat with antibacterial medication. Dropsy: swollen abdomen with raised scales (pinecone), often fatal; quarantine and treat early.
Oxygen Deprivation
Fish gasping at the surface or hanging near the filter outlet usually indicates low dissolved oxygen. Causes: water too warm, filter not running, overstocking, or surface covered by floating plants. Fix by increasing surface agitation (raise filter outlet, add an air stone) and reducing temperature.
Poisoning
Sudden deaths with no disease symptoms often indicate poisoning. Common culprits: soap or detergent residue, copper (fertilisers, old pipes, medications), chlorine (tap water without dechlorinator), aerosol sprays near the tank, or new décor leaching toxins.
Compatibility Issues
Some fish simply shouldn't be kept together. Research every species before adding. Common incompatibilities: fin-nippers with long-finned fish, predators with small fish, and aggressive territorial species with peaceful community fish.
When to Call a Vet
If fish are dying despite good water parameters and no obvious disease, or a disease isn't responding to treatment within 5–7 days, consult an aquatic vet. Specialist fish vets are more widely available than most people realise.
Find liquid test kits, water conditioners, fish medications, and air pumps in our Water Care range.
