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 50+ common freshwater species. Each has a bioload weight based on adult size and activity level.
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. A 10-inch Oscar produces far more waste than ten 1-inch Neon Tetras. TankMate uses a bioload weighting system that factors in adult body mass, activity level, and waste production per species. Combined with your filtration type and tank volume, you get a realistic stocking gauge — not a vague rule of thumb. 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. A 10-inch oscar produces far more waste than ten 1-inch neon tetras, yet both equal '10 inches.' Bioload-based calculations weight each species by actual waste output, activity level, and adult size, producing much more accurate stocking recommendations.
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 can support roughly 10-15 small community fish (tetras, rasboras, corydoras) with adequate filtration. 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).