Ammonia

A nitrogen compound (NH₃/NH₄⁺) released by fish waste and decaying matter. Acutely toxic to fish at concentrations above ~0.25 ppm. In a cycled tank, ammonia should always read 0.

Ammonia exists in water as two interconverting forms: NH₃ (un-ionized, highly toxic) and NH₄⁺ (ionized, far less toxic). The ratio depends on pH and temperature — higher pH and warmer water shift more of the total toward the toxic NH₃ form. That's why an ammonia spike in a high-pH tank is more dangerous than the same numeric reading in a low-pH tank.

In a properly cycled tank, ammonia-eating bacteria in the filter convert it as fast as fish produce it, so test kits read 0. Any non-zero reading on an established tank signals either: (a) the cycle has crashed (e.g. after a filter clean), (b) the tank is overstocked relative to filter capacity, or (c) something has died unnoticed.

Liquid test kits (API Master, Salifert, etc.) are more accurate than test strips for ammonia.

Beginner-safe rule

Beginner-safe rule: if ammonia reads above 0.25 ppm, do a 50% water change immediately and investigate the cause. Repeat daily until it returns to 0. Don't add more fish during an ammonia spike.