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Closed Loop · Step 1

Fish feed the water — the nitrogen origin story

In any recirculating system, the fish are the nitrogen source. They eat protein, oxidise the amino acids for energy, and excrete the leftover nitrogen — mostly across their gills, not in their faeces — as un-ionised ammonia (NH₃). Understanding how much, how fast, and at what pH it becomes toxic is the first thing any aquaponic operator needs to get right.

Where the nitrogen actually comes from

Fish are ammoniotelic: roughly 75–90 % of nitrogenous waste leaves the body as ammonia diffusing directly across the gill epithelium, with the remainder excreted as urea and uric acid. Only a small fraction comes from undigested faecal protein.

NRC, 2011
Timmons & Ebeling, 2013

A widely used engineering rule from recirculating aquaculture: about 0.03 kg of total ammonia-nitrogen (TAN) is produced per 1 kg of feed fed per day (i.e. ~30 g TAN / kg feed). Higher-protein diets push this higher.

Timmons & Ebeling, 2013
Rakocy et al., 2006 (UVI/SRAC 454)

UVI feed-rate ratio (g feed / m² raft / day)Leafy greens60–100 g/m²/dayFruiting crops80–120 g/m²/day050100150
UVI design heuristic: ~60–100 g feed / m² raft / day for leafy greens, ~80–120 g for fruiting crops. From Rakocy et al. (2006).

Why ammonia is dangerous — and pH matters more than people think

Total ammonia in water exists in equilibrium between un-ionised NH₃ (highly toxic) and ionised NH₄⁺ (relatively benign). The split is driven by pH and temperature: at pH 7.0 and 25 °C only ~0.6 % of TAN is NH₃, but at pH 8.5 and 25 °C that jumps to ~15 %. A single point of pH drift can turn a safe tank into a lethal one.

Francis-Floyd et al., 2009
Timmons & Ebeling, 2013

Chronic safe exposure for warm-water species like Nile tilapia is generally cited as < 0.05 mg/L NH₃-N; acute LC₅₀ values are commonly in the 1–3 mg/L NH₃-N range. This is why aquaponic operators measure pH and TAN at least weekly during cycling and after every stocking change.

El-Sayed, 2020 (FAO)
Francis-Floyd et al., 2009

What the fish need from you in return

  • Dissolved oxygen ≥ 5 mg/L at all times for tilapia and African catfish (≥ 6 mg/L is safer for growth).
  • Stocking density tuned to your filtration, not your tank volume — UVI commercial design uses ~60–77 kg/m³ in the rearing tanks at harvest.
  • Feed input matched to the plant area downstream — the feed-rate ratio (Step 2) is what keeps the loop balanced.

“The most reliable way to size an aquaponic system is from the feed rate, not the fish count or plant count. Feed input drives ammonia production, which drives plant nutrient supply.”

— Rakocy et al., 2006 (UVI/SRAC 454)

≈ 30 g
TAN per kg feed
Timmons & Ebeling, 2013
75–90 %
of N excreted via gills
NRC, 2011
< 0.05 mg/L
Chronic safe NH₃-N
Francis-Floyd et al., 2009
≥ 5 mg/L
DO for tilapia & catfish
US EPA, 1986

References

  1. NRC, 2011. National Research Council (2011). Nutrient Requirements of Fish and Shrimp. The National Academies Press, Washington, D.C. link
  2. Timmons & Ebeling, 2013. Timmons, M.B., Ebeling, J.M. (2013). Recirculating Aquaculture, 3rd ed. Ithaca Publishing.
  3. Rakocy et al., 2006 (UVI/SRAC 454). Rakocy, J.E., Masser, M.P., Losordo, T.M. (2006). Recirculating Aquaculture Tank Production Systems: Aquaponics — Integrating Fish and Plant Culture. SRAC Publication No. 454. link
  4. Francis-Floyd et al., 2009. Francis-Floyd, R., Watson, C., Petty, D., Pouder, D.B. (2009). Ammonia in Aquatic Systems. UF/IFAS Extension FA16. link
  5. El-Sayed, 2020 (FAO). El-Sayed, A.-F.M. (2020). Tilapia Culture, 2nd ed. Academic Press / FAO.
  6. US EPA, 1986. U.S. Environmental Protection Agency (1986). Ambient Water Quality Criteria for Dissolved Oxygen. EPA 440/5-86-003.

Every number on this page is sourced to one of the references above. Nothing is AI-generated or unverified — if a claim can't be traced to a peer-reviewed paper, FAO/UVI technical report or major university extension, it doesn't appear here.