Free Ammonia Calculator
Your test kit reading on its own does not tell you whether your fish are in danger. Only part of that ammonia is the toxic form, and how much depends entirely on pH and temperature.
API and most US hobby kits report total ammonia. Seachem, Salifert and most lab reports use nitrogen (NH₃-N). If the bottle says "NH₃-N" or "as N", pick the second option — it changes the answer by about 22%.
Thresholds here are for koi and other cyprinids, which tolerate ammonia better than trout or salmon but far less well than the internet generally assumes. Damage from free ammonia is cumulative and starts at the gills long before fish look unwell. Salt in the pond shifts the balance very slightly toward the safer form, which this calculator ignores — so if you are running salt, the real figure is marginally better than shown, never worse.
