Suddenly losing fish — what to do

First steps

1. Remove any dead fish immediately to avoid ammonia spikes and disease spread.
2. Test ammonia, nitrite, nitrate, and temperature. Write the results down.
3. Do a partial water change (20–30%) with dechlorinated water.
4. Check the remaining fish for signs of illness: spots, clamped fins, gasping, hiding.

If ammonia and nitrite are zero

Good readings rule out a cycle crash, but fish can still die from other causes.

• Disease: Internal parasites, bacterial infections, or viruses can kill without obvious signs. Quarantine sick fish if you have a hospital tank. See our top 10 fish illnesses guide.
• Aggression: Bullying or territorial fish can stress or injure others. Watch for chasing, nipping, or one fish dominating food and hiding spots.
• Sudden stress: Loud noise, tank moves, new tank mates, or temperature swings can trigger shock.
• Old age or hidden illness: Sometimes fish die from age or conditions that were not visible.

What to do when readings look good

Reduce feeding for a few days. Observe the tank closely. If more fish show symptoms, isolate them and consider treatment. If aggression is likely, add hiding spots or rehome the aggressor. Keep up small water changes to keep things stable.

When to suspect water quality

Even if ammonia and nitrite are zero, high nitrate, low oxygen, or chlorine from untreated tap can cause problems. Test nitrate; aim under 40 ppm for most fish. Ensure good surface agitation and always use dechlorinator.

Prevention

Quarantine new fish before adding to the main tank. Avoid overstocking and overfeeding. Keep a consistent maintenance routine and track parameters so you spot changes early.