Set Your Tank
Enter tank volume and filtration type. Better filtration increases your tank's bioload capacity.
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An aquarium stocking calculator determines bioload capacity by weighting each species by its actual waste output rather than the inaccurate inch-per-gallon rule. Select from 40+ freshwater species, enter tank volume and filtration, and see stocking percentage with compatibility warnings.
Tank Setup
Add Species
Search and add species above
Stocking Level
Add fish to see stocking level
Total Fish0
Total Inches0"
Bioload Score0
Water Changes—
How It Works
Add Species
Search from 40+ common freshwater species. Each carries a bioload weight derived from adult body mass, body form and diet — not from length.
Read the Gauge
The bioload gauge shows your stocking level: green (0–75%), yellow (75–100%), red (100%+). Stay in the green.
Maintenance Guide
Based on your bioload, TankMate recommends water change frequency to keep your fish healthy.
Beyond "Inch Per Gallon"
The old "one inch of fish per gallon" rule fails for anything beyond small community tetras. Ten neon tetras total 15 inches and a single adult oscar totals 12 — the inch rule calls the oscar the lighter load, while the oscar produces more than ten times their waste. TankMate weights each species by adult body mass rather than length: waste output tracks mass, so doubling a fish's length multiplies its load roughly fourfold, and body form and diet adjust it from there. The large end is calibrated against published single-specimen minimums — one adult oscar or common pleco fills a 75-gallon canister tank on its own. Every calculation runs locally in your browser.
Frequently Asked Questions
What is the inch per gallon rule and is it accurate?
The 'one inch of fish per gallon' rule is a rough guideline that breaks down quickly. Ten neon tetras total 15 inches and a single adult oscar totals 12, so the inch rule calls the oscar the lighter load — while the oscar produces more than ten times their waste. Bioload-based calculations weight each species by adult body mass, body form and feeding habits, which is what actually drives ammonia production.
What is bioload in an aquarium?
Bioload is the total waste output from all fish and organisms in the tank — ammonia from respiration, uneaten food, and decomposing organic matter. Filters convert ammonia to nitrite to nitrate (the nitrogen cycle). When bioload exceeds your filter's biological capacity, ammonia spikes and fish die. Bioload depends on fish species, size, feeding habits, and temperature.
How many fish can I put in a 20-gallon tank?
A 20-gallon tank with hang-on-back filtration supports roughly 10-12 schooling fish under 2 inches — neon tetras, ember tetras, harlequin rasboras. Heavier-bodied fish draw more capacity per fish: six bronze corydoras use more of a 20-gallon than ten neon tetras. Stock gradually — add 3-4 fish at a time with 2 weeks between additions to let the biological filter catch up. Use a filter rated for at least 1.5× your tank volume (30-gallon filter for a 20-gallon tank).