Test the water for ammonia immediately
Perform a partial water change with dechlorinated water
Match the new water temperature to the tank
Use a water conditioner that detoxifies ammonia
Stop overfeeding and remove uneaten food
Remove dead fish, plants, or other decaying matter
Clean the filter media gently in tank water
Check that the filter is working properly
Add beneficial bacteria starter if needed
Increase aeration with an air stone or stronger surface agitation
Reduce fish stocking if the tank is overcrowded
Feed fish less until ammonia is under control
Vacuum the substrate to remove waste
Repeat water changes as needed until ammonia is zero
Monitor ammonia, nitrite, and nitrate daily until stable