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Closed Loop · Step 3

Clean water returns to fish — polishing, oxygen & recirculation

The third leg of the loop is the part most beginners under-design: actually getting clean, well-oxygenated water back to the fish. This is where mechanical filtration, plant uptake and aeration come together — and where total water use drops to a small fraction of soil farming.

What 'clean' actually means in a recirculating system

Water returning to the fish tank should be < 1 mg/L TAN, < 1 mg/L NO₂⁻-N, ≥ 5 mg/L dissolved oxygen and within 1 °C of the tank temperature. Total suspended solids should be < 25 mg/L — anything higher coats gills and biofilter media.

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

5 mg/L — minimum for tilapia & catfish5°C15°C25°C35°C681012Freshwater O₂ saturation vs temperature
Dissolved-oxygen saturation in fresh water falls as temperature rises — the design ceiling for stocking density. Data: USGS / Mortimer (1981).

Why dissolved oxygen sets the ceiling on stocking density

Oxygen solubility in fresh water is roughly 8.3 mg/L at 25 °C and only 7.5 mg/L at 30 °C — and tilapia at full stocking can consume 250–400 mg O₂ per kg of fish per hour. Either you aerate aggressively (air stones, venturi, low-head oxygenators) or you cap density. There is no third option.

Timmons & Ebeling, 2013
El-Sayed, 2020 (FAO)

Water use vs. soil agriculture

Properly cycled aquaponic systems lose water mainly to plant transpiration and evaporation. Make-up water typically runs 1–3 % of system volume per day — well-documented field measurements show total water use of 2–10 % of what an equivalent soil crop would consume.

Somerville et al., 2014 (FAO 589)
Goddek et al., 2019

Closing the loop responsibly

  • Mechanical filter (radial-flow / swirl / drum) BEFORE the biofilter — solids choke nitrification.
  • Sump tank as the lowest point so a pump failure can't drain the fish tank.
  • Auto top-up from a buffered reservoir, not straight municipal water (chlorine/chloramine kill biofilter bacteria).
  • Weekly tests: pH, TAN, NO₂⁻, NO₃⁻, DO, temperature. Monthly: alkalinity, Fe, EC.
1–3 %
Daily top-up volume
Somerville et al., 2014 (FAO 589)
≈ 90 %
Less water than soil
Goddek et al., 2019
≥ 5 mg/L
DO returning to fish
US EPA, 1986
< 25 mg/L
TSS to biofilter
Timmons & Ebeling, 2013

References

  1. Timmons & Ebeling, 2013. Timmons, M.B., Ebeling, J.M. (2013). Recirculating Aquaculture, 3rd ed. Ithaca Publishing.
  2. 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
  3. Somerville et al., 2014 (FAO 589). Somerville, C., Cohen, M., Pantanella, E., Stankus, A., Lovatelli, A. (2014). Small-scale aquaponic food production. FAO Fisheries and Aquaculture Technical Paper No. 589. Rome, FAO. link
  4. Goddek et al., 2019. Goddek, S., Joyce, A., Kotzen, B., Burnell, G.M. (Eds.) (2019). Aquaponics Food Production Systems. Springer (open access). link
  5. US EPA, 1986. U.S. Environmental Protection Agency (1986). Ambient Water Quality Criteria for Dissolved Oxygen. EPA 440/5-86-003.
  6. El-Sayed, 2020 (FAO). El-Sayed, A.-F.M. (2020). Tilapia Culture, 2nd ed. Academic Press / FAO.

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.