The nitrogen cycle in a pond
Every kilogram of feed you add brings ~40 g of nitrogen into the pond. Uneaten feed, faeces, and gill excretion all convert to ammonia (NH₃/NH₄⁺). Nitrifying bacteria oxidise ammonia to nitrite (NO₂⁻), then nitrate (NO₃⁻). Algae reabsorb nitrate to close the loop — when it works.
Ammonia toxicity — TAN vs unionised NH₃
Total Ammonia Nitrogen (TAN) is what your test kit measures. The toxic fraction is unionised NH₃, which rises sharply above pH 8.5 and above 30 °C. Keep TAN under 1 mg/L and unionised NH₃ under 0.05 mg/L.
Nitrite toxicity — the brown-blood risk
Nitrite binds haemoglobin/haemocyanin, forming methaemoglobin (fish) or blocking oxygen transport (shrimp) — the classic "brown blood" syndrome. Keep NO₂⁻ under 1 mg/L for shrimp, 0.5 mg/L for fish.
Testing frequency and thresholds
Test TAN and nitrite 2–3× per week during grow-out, daily after DOC 60 or if feed rates exceed 30 kg/ha/day. Log both — a rising nitrite trend with steady TAN means the second nitrifier population is lagging.
Building and protecting the biofilter
Nitrifiers colonise pond substrate, biofloc particles and artificial substrates. Protect them by avoiding antibiotic dumps, avoiding sudden salinity swings, and maintaining DO above 4 mg/L — nitrification is oxygen-hungry (4.5 g O₂ per g NH₃-N).
Response when TAN or NO₂⁻ spikes
- Reduce feed by 20–30% for 48 h.
- For nitrite spikes, add NaCl at 6:1 Cl⁻:NO₂⁻ ratio.
- Add fresh Bacillus probiotic to seed the biofilter.
- Increase aeration — nitrification demands DO.
- If both spike, partial water exchange (20–30%) buys time.