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Aquaponic Stocking Density.
Aquaponic Stocking Density Calculator
Balance fish biomass with grow-bed area and biofilter capacity. Three numbers must agree, or the system collapses.
The aquaponic balance equation
Three components must be in balance:
| Component | Job | Sizing rule |
|---|---|---|
| Fish | Produce ammonia (waste) | 0.5-1.0 lb fish per gallon of fish tank water (max ~2 lb/gal in well-aerated systems) |
| Biofilter (mostly the grow beds) | Convert ammonia → nitrite → nitrate via nitrifying bacteria | 1 sq ft grow bed per 0.5-1 lb fish biomass |
| Grow beds (plants) | Consume nitrate; complete the loop | UVI ratio: 60-100 g fish feed per sq m of grow bed daily |
If fish biomass exceeds biofilter capacity, ammonia/nitrite spike and fish die. If grow bed exceeds plant uptake, system runs nitrogen-poor and plants stunt. Balance is everything.
Species comparison
| Species | Temp range | Time to harvest | Stocking density | Edibility |
|---|---|---|---|---|
| Tilapia | 72-86°F (warm) | 6-9 months to 1.5 lb | 0.5-1.0 lb/gal | Excellent; mild flavor; market-staple |
| Rainbow Trout | 50-65°F (cold) | 9-12 months | 0.5-0.75 lb/gal | Excellent; needs chiller in summer most US |
| Yellow Perch | 60-75°F (cool) | 12-18 months | 0.4-0.6 lb/gal | Excellent; more challenging breeding |
| Channel Catfish | 70-85°F (warm) | 10-14 months | 0.6-1.0 lb/gal | Good; muddy flavor unless purged |
| Bluegill | 60-80°F (cool) | 12-18 months | 0.3-0.5 lb/gal | Good; small fish; good for hobby |
| Koi / Goldfish | 50-80°F (wide) | Display only | 0.2-0.4 lb/gal | Not eaten; just produces nutrients |
Common problems
- System cycling phase (first 4-6 weeks): Bacteria establish slowly. Stock fish lightly, monitor ammonia + nitrite daily. Don't add many fish before nitrite has dropped to zero.
- pH drift: Nitrification produces acid. pH slowly drops over months. Buffer with calcium carbonate (oyster shell) or potassium hydroxide; never below 6.5 (kills bacteria) or above 7.5 (locks plant nutrients).
- Iron deficiency: Fish feed has minimal iron. Plants almost always need iron supplementation in aquaponics — chelated iron (FeDTPA, FeEDDHA) every 2-4 weeks.
- Phosphorus deficiency in fruiting crops: Fish feed is N-rich and P-poor. Tomatoes/peppers may need P supplementation as fruiting starts.
- Solids handling: Fish solids must be removed (settling tank or radial flow filter) before they hit grow beds, or anaerobic decomposition kills roots and fish.
Free under CC BY 4.0. Cite as "OAT Aquaponic Stocking Density Calculator (openagriculturetechnology.com)". Sizing rules from UVI (Univ. of Virgin Islands) standards and contemporary research.