Aquaponics Fish to Plant Ratio & Ammonia Calculator
Enter your fish and this tells you how many plants to grow, how big a biofilter you need, and whether your ammonia is safe — updating live as you type.
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How this calculator works
Aquaponics balances three things at once: the fish produce ammonia from their feed, nitrifying bacteria in the biofilter convert that ammonia to nitrate, and the plants absorb the nitrate. Get the balance wrong and either the fish are poisoned by ammonia or the plants starve. This tool works from your daily feed — which it can estimate from the number and size of your fish — and uses published aquaculture formulas to size all three parts of the loop, and to flag ammonia toxicity before it becomes dangerous.
1. Ammonia production from feed (Timmons & Ebeling)
Fish excrete nitrogen from digested protein as Total Ammonia Nitrogen (TAN). The widely used stoichiometric estimate is:
TAN (g/day) = daily feed (g) × (protein % ÷ 100) × 0.092
The constant 0.092 reflects that roughly 9.2% of the protein consumed is excreted as ammonia-nitrogen. Protein content depends on the fish: tilapia and other omnivores usually eat 25–35% protein feed, while carnivorous trout or salmon need up to 55%. As a check, 1,000 g of 40% feed produces about 36.8 g of TAN per day.
2. Why un-ionized ammonia is the real danger (Emerson, 1975)
TAN is a mix of toxic un-ionized ammonia (NH₃) and relatively harmless ammonium (NH₄⁺). Only the un-ionized fraction crosses the fish's gills and causes harm, and that fraction climbs steeply with pH and temperature. The calculator uses the Emerson equilibrium equations:
pKₐ = 0.09018 + 2727.92 ÷ T (T in Kelvin)
NH₃ fraction = 1 ÷ (10^(pKₐ − pH) + 1)
Un-ionized ammonia = TAN × NH₃ fraction
The effect is dramatic. At 20 °C and pH 7.0 only about 0.4% of TAN is toxic, but at pH 8.0 it is nearly 4% — a tenfold jump. In warm water at 30 °C and pH 8.5, over 20% is toxic. That is why the same ammonia reading can be perfectly safe or lethal depending on your water. Aim to keep un-ionized ammonia below 0.05 mg/L.
| Temp | pH 7.0 | pH 7.5 | pH 8.0 | pH 8.5 |
|---|---|---|---|---|
| 15 °C | 0.27% | 0.85% | 2.67% | 7.97% |
| 20 °C | 0.40% | 1.24% | 3.83% | 11.18% |
| 25 °C | 0.57% | 1.77% | 5.38% | 15.25% |
| 30 °C | 0.80% | 2.48% | 7.46% | 20.30% |
3. Sizing your biofilter
Nitrifying bacteria live on the surface of biofilter media, so the media you need depends on its specific surface area (SSA) and how fast it nitrifies:
Media volume (m³) = TAN ÷ (nitrification rate × SSA)
A 30–50% safety margin is then added for cold snaps, overfeeding and biological lag. Below 15 °C the calculator also reduces the nitrification rate, because bacterial activity drops 40–60% in cold water and you need more media to keep up.
| Media | SSA (m²/m³) | Rate (g/m²/day) |
|---|---|---|
| K1 Kaldnes (MBBR) | 500–800 | 0.60 |
| Expanded clay (Hydroton) | 300–500 | 0.35 |
| Pea gravel | ~200 | 0.20 |
| Fluidized sand bed | 5,000–11,500 | 0.06–0.20 |
4. The fish-to-plant ratio (UVI feeding-rate method)
Rather than crude weight ratios, the University of the Virgin Islands (Rakocy et al.) method matches the plant area to the daily feed, which keeps nitrate from building up:
Grow-bed area (m²) = daily feed (g) ÷ feeding-rate ratio (g/m²/day)
| Crop | Feeding rate (g/m²/day) |
|---|---|
| Leafy greens & herbs (lettuce, basil, spinach) | 20–40 |
| Fruiting vegetables (tomato, cucumber, pepper) | 50–80 |
How to use it
Pick "I know my fish" and enter how many fish you have, their average weight, and how much you feed. The tool estimates your daily feed and instantly shows the grow-bed area, biofilter media and ammonia status — no button to press. Prefer to enter feed directly? Switch to "I know my feed." If you have a test-kit ammonia reading, add it under Advanced options for an exact toxicity figure.
Important: these are planning estimates based on published averages. Real systems vary with stocking density, solids removal, dissolved oxygen and feed quality. Ammonia can harm fish quickly — always confirm with a water test kit and monitor your system directly. This tool is for education and planning, not a substitute for hands-on management.
FAQ
What is the ideal fish-to-plant ratio in aquaponics?
Modern systems size plants by feeding rate, not crude weight. Aim for 20–40 g of feed per m² of grow bed per day for leafy greens, and 50–80 g/m²/day for fruiting vegetables. Divide your daily feed by that rate to get the grow-bed area.
What ammonia level is safe in aquaponics?
Keep un-ionized ammonia (NH₃) below about 0.05 mg/L, which usually means total ammonia nitrogen (TAN) under roughly 0.5 mg/L. The safe TAN level drops as pH and temperature rise.
How does pH and temperature affect ammonia toxicity?
Only the un-ionized fraction of ammonia is highly toxic, and it rises steeply with pH and temperature — from about 0.4% of TAN at 20 °C/pH 7 to over 20% at 30 °C/pH 8.5. The same reading can be safe or lethal depending on your water.
How much biofilter media do I need for tilapia?
Find daily ammonia from feed and protein, then divide by the media's nitrification rate × its surface area, and add a 30–50% margin. For example, 500 g of 32% feed makes about 14.7 g TAN/day, needing roughly 64 L of K1 (MBBR) media at a 30% buffer.