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FurCalc

Aquarium Stocking Calculator — Is My Tank Overstocked?

Is your freshwater aquarium understocked, fine, or overstocked? This calculator uses a simplified bioload model — combining adult size, species metabolism rate and waste output — across 14 common community-tank species, then compares the total to your tank’s filter and gas-exchange capacity. See National Aquarium care guidance for species-specific tank-mate compatibility before you finalize stock.

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Filter quality substantially shifts safe stocking — planted tanks absorb nitrate biologically.

Fish list
Bioload
20 %of tank capacity
understocked
Bioload model simplifies complex factors — filtration type, water change frequency, planted vs bare tank all matter. Use AqAdvisor.com for deeper analysis.

Inch-per-gallon rule vs bioload model: why size alone misleads

AspectInch-per-gallon ruleBioload model (this tool)
InputsAdult lengthLength + species metabolism + activity
Works for small tetrasYesYes
Works for goldfish / cichlidsNo — under-estimates badlyYes — accounts for ~3× waste rate
Filter capacity checkNoYes
Surface-area / gas exchangeNoYes (long tanks beat tall tanks)

Aquarium stocking terms

Bioload
Total ammonia and waste output a fish produces — driven by size, diet, and metabolic rate.
Filter turnover (GPH)
Gallons of tank water cycled through the filter per hour; healthy community tanks target 4-10× tank volume.
Surface agitation
The disturbance that drives gas exchange — more important for stocking limits than tank height alone.
Stocking percentage
Estimated bioload as a fraction of filter + tank capacity. Keep below 80% for resilience to skipped maintenance.

Frequently Asked Questions

Is the 'inch per gallon' rule accurate?

No. The old 'one inch of fish per gallon' rule fails for large fish: a 12-inch Oscar produces far more waste than twelve 1-inch neons. Modern stocking uses bioload (waste output), surface area for gas exchange, and filtration capacity.

How do I know if my tank is overstocked?

Symptoms: cloudy water, ammonia/nitrite readings >0, high nitrate (>40 ppm) between water changes, fish gasping at the surface, frequent disease outbreaks. Get a liquid test kit (API Master) and test weekly.

Can I keep a fancy goldfish in a 10 gallon tank?

No. Fancy goldfish need 20 gallons for the first fish and +10 gallons per additional fish. Common goldfish and comets grow 12+ inches and need ponds or 75+ gallon tanks.

How many neon tetras can I keep in a 10 gallon tank?

6–8 neon tetras with minimal other stock. Neons are schooling fish and need at least 6 to reduce stress; a 20-gallon tank comfortably holds 10–12 plus compatible tank mates.

What is bioload?

The amount of waste (primarily ammonia from gills and poop) fish produce. It depends on fish size, diet, activity level and species metabolism. Fancy goldfish have ~3× the bioload of similar-sized tropicals.

Sources & References

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Common mistakes to avoid

  • One-inch-per-gallon rule — outdated. Big-bodied fish produce way more waste than long thin ones.
  • Stocking all at once — even a cycled tank needs gradual additions (2-3 fish/week) to let bacteria catch up.
  • Ignoring adult size — “3-inch” fish at the store may grow to 10 inches (common plecos).

Pro tips

  • Surface area (L × W) matters more than volume for gas exchange. Tall tanks hold fewer fish than shallow tanks of same volume.
  • Schooling fish need 6+ of their own species, not crammed singles.
  • Test water for 4 weeks post-stocking to verify filter keeps up.