What to do when ammonia spikes

Why ammonia spikes

Ammonia comes from fish waste, uneaten food, and decaying matter. In a cycled tank, beneficial bacteria convert it to nitrite and then nitrate. When the tank isn’t cycled, the filter is disrupted, or waste outstrips the bacteria (overfeeding, overstocking), ammonia can spike. Even a reading above zero is dangerous.

Immediate steps

1. Test — Confirm with your test kit. Check ammonia, and also nitrite and pH. Log the result (e.g. in App-aquatic) so you can track progress.
2. Water change — Do a large partial water change (30–50%) with dechlorinated water at a similar temperature. This dilutes ammonia fast and gives fish relief.
3. Stop feeding — Skip feeding for 24–48 hours so no extra waste is produced. Fish can handle a short fast.
4. Don’t remove the filter — Your filter holds beneficial bacteria. Rinsing media in tap water or removing it can make things worse. If you must rinse, use old tank water.

Find and fix the cause

Common causes: tank not fully cycled (see how long to cycle), overfeeding (see how much to feed), overstocking (see overstocking), a dead fish or rotting plant, or a new filter that hasn’t established bacteria. Fix the underlying issue and keep testing until ammonia stays at zero.

Quick takeaways

• Test first; then do a large water change and stop feeding for a day or two.
• Never rinse filter media in tap water — use tank water if needed.
• Fix the cause (cycle, feeding, stocking) and keep testing until ammonia is zero.