What Causes Ammonia Spikes in Freshwater Tanks?

Ammonia should read zero in a cycled tank. Spikes usually mean the biofilter was overwhelmed: too many new fish at once, heavy overfeeding, a dead fish decomposing, filter media washed in chlorinated tap water, or a long power outage.

Test immediately if fish gasp, hang near the surface, or lose appetite. A partial water change of 30% to 50% with dechlorinated water of matched temperature is the first response. Stop feeding until ammonia returns to zero. Add bottled bacteria only if you trust the product; it is not a substitute for finding the cause.

Log each reading and change in ReefDiary. A spike that follows a big feeding weekend or a new fish shipment is a clue for next time.