Ammonia: The Invisible Killer
Every kilogram of feed entering a fish pond ultimately produces about 35 grams of ammonia nitrogen. In a closed system, this ammonia accumulates relentlessly — and un-ionized ammonia (NH₃) is one of the most toxic substances fish encounter. At concentrations as low as 0.05 mg/L, it damages gill epithelia, suppresses immune function, and inhibits growth. At 0.5 mg/L, it kills within hours.
The pH-Ammonia Nexus
Total ammonia nitrogen (TAN) exists in a pH-dependent equilibrium between toxic NH₃ and relatively harmless NH₄⁺. At neutral pH (7.0), less than 1% of TAN is in the toxic form. But at pH 9.0 — common in ponds with afternoon algal blooms — over 30% becomes toxic NH₃. This means a pond can go from safe to lethal without any change in total ammonia, simply because photosynthesis drove pH upward during the day.
Nitrification: Nature's Detox
Two groups of autotrophic bacteria perform the critical detoxification. Nitrosomonas oxidize ammonia to nitrite (NO₂⁻), which is itself toxic to fish at concentrations above 1 mg/L. Nitrobacter then convert nitrite to nitrate (NO₃⁻), which fish tolerate at levels up to 200 mg/L. These bacteria colonize biofilter media and require oxygen, alkalinity, and stable conditions to maintain active populations.
Modeling the Nitrogen Cascade
This simulation tracks nitrogen through the full ammonia → nitrite → nitrate pathway using Monod kinetics for bacterial conversion rates. Adjust feed loading, pH, biofilter capacity, and water exchange to find the equilibrium concentrations. Watch how pH amplifies toxicity and how biofilter capacity creates a threshold below which the system cannot cope with ammonia production.