FishTankWhy are my fish dying?
Losing fish is upsetting, and it's even worse when you don't know why. Most deaths come down to a short list of causes, and the water is usually the first place to look. Here's what to do right now, then how to find the cause.
What to do right now
- Remove any dead fish straight away. A body rotting in the tank raises ammonia quickly.
- Test the water for ammonia, nitrite, nitrate and pH. Write the numbers down.
- Check the temperature with a thermometer you trust, not the dial on the heater.
- Do a 30–50% water change if ammonia or nitrite is above zero or nitrate is high. Use water conditioner and match the temperature.
- Skip feeding for a day while you work out what's wrong.
- Look closely at the fish that are left. Spots, torn fins, fast breathing and hiding are all clues.
The most common causes
1. The tank wasn't cycled
The most common cause of all. A new tank doesn't have enough good bacteria yet to break down fish waste, so ammonia and nitrite build up. Fish start dying in the first few weeks. The fix: frequent water changes now, and learn how to cycle a tank so it doesn't happen again.
2. Ammonia from overfeeding or too many fish
Uneaten food rots, and every fish adds waste. Too much of either and the filter can't keep up. See ammonia in a fish tank and how often to feed fish.
3. Untreated tap water
Most tap water has chlorine or chloramine in it, which burns fish gills and kills the bacteria in your filter. Every bucket of new water needs conditioner.
4. The wrong temperature
A heater that's stuck on can cook a tank in a few hours, and a broken one lets it go cold. Tropical fish kept too cold get sick more easily. Use a separate thermometer and check it every day.
5. Sudden changes
A huge water change after months without one, water at a very different temperature, or new fish dumped straight in without acclimating can all shock fish. Change water little and often instead.
6. Disease from new fish
If deaths started a week or two after you added new fish, they may have brought something with them. Ich (white spots) is the most common. See how to treat ich, and quarantine new fish in future.
7. Bullying or the wrong tankmates
Torn fins, fish hiding all the time, or only one kind of fish dying point to bullying. Some fish nip fins, and some eat smaller fish. The stocking calculator warns you about common bad pairings.
8. Not enough oxygen
Fish hanging at the top and gulping in the morning is a sign. See fish gasping at the surface.
9. Something got into the water
Soap left in a bucket, hand lotion, air freshener, bug spray, cleaning sprays used near the tank, and decorations or rocks that weren't made for aquariums can all poison fish. If all the fish are struggling at once and the tests look normal, think about what has been used near the tank.
Reading the clues
- New tank, several fish dying: ammonia or nitrite from an uncycled tank.
- All the fish gasping at once: low oxygen, ammonia, or something toxic in the water.
- White spots like salt: ich.
- Torn or missing fins: fin nipping or bullying, or fin rot from dirty water.
- Only one fish died and the rest are fine: it may have been weak from the store, or old. Test anyway to be sure.
What you need to find the problem
Paid links: if you buy through these Amazon links, I earn a small commission at no extra cost to you.
- Test kitAPI Freshwater Master Test KitThe first thing to reach for. Ammonia and nitrite should both read zero.
- Water conditionerSeachem Prime (500 ml)Treat the new water for an emergency water change.
- ThermometerZacro digital thermometer (2 pack)A separate thermometer tells you if the heater is working.
- Bottled bacteriaTetra SafeStart PlusLive bacteria that help a new or struggling tank handle fish waste.
Related: Ammonia in a fish tank · How to cycle a new fish tank · White spots on fish (ich)