Aquarium Stocking Calculator
Build your stocking plan species by species. The calculator combines adult size, activity, schooling needs, temperament, and filtration to give you a realistic planning status —not a one-size-fits-all rule.
Your Fish
- Neon Tetra~1.2" adult · keep 6+
Planning status is a guideline, not a guarantee. Watch your water parameters and adjust gradually.
How It Works
Instead of the crude "one inch per gallon" rule, this calculator scores each species' bioload from its adult size and activity level, sums it across your plan, and compares it against your tank's capacity adjusted for filtration. It then runs independent welfare checks: minimum tank size per species, schooling group sizes, and temperament conflicts between species. The result is a Planning Status with the specific issues to fix —because stocking is a planning exercise, not a pass/fail exam.
Formula
Bioload = Σ (Adult Size × Activity Factor × Quantity)
Capacity = Tank Gallons × Filtration Factor
Planning Status = f(Bioload ÷ Capacity, Rule Checks)
Example
A 20 gallon tank with standard filtration: 8 neon tetras (1.2", high activity) plus 1 betta (2.5", low activity) scores about 14 bioload points against a capacity of 20 —a comfortable Moderate plan with no rule violations. Swap the neons for 3 angelfish and the calculator immediately flags the 30 gallon minimum.
Important Limitations
- Bioload points are a planning heuristic calibrated from hobby guidelines, not a lab measurement.
- Individual temperament varies —especially bettas and dwarf gouramis.
- Breeding livebearers (guppies, mollies, platies) will push a plan toward Heavily Stocked over time.
- Always cycle the tank fully before adding the full plan.
FAQ
How many fish can I put in a 20 gallon tank?
There is no single number —it depends on the species' adult size, activity level, and behavior. A 20 gallon community might hold 8 neon tetras and a betta, or 6 harlequin rasboras and a group of corydoras. The calculator above scores your exact plan instead of guessing with rules of thumb.
Is the one inch of fish per gallon rule correct?
Only loosely. A 10-inch oscar does not equal ten one-inch neon tetras —body mass, waste output, and activity scale disproportionately. Use adult size (not purchase size), and treat any rule as a starting point, not a verdict.
What does a stocking level of Moderate mean?
Moderate means your plan fits comfortably within your tank's capacity with a normal maintenance routine. Heavily Stocked means it can work but demands excellent filtration and more frequent water changes. Overstocked means the plan needs to change.
Why does schooling matter for stocking?
Schooling fish (tetras, rasboras, corydoras) are stressed and shy when kept in groups below their minimum. Six stressed fish create more problems than twelve comfortable ones —always plan for the full school size.
Does a bigger filter let me keep more fish?
A heavily over-sized filter or a heavily planted tank raises your effective capacity somewhat —the calculator accounts for that. But filtration never substitutes for tank size: swimming room and territory matter independently of water cleanliness.
When should I add fish to a new tank?
After the tank has fully cycled (0 ppm ammonia, 0 ppm nitrite, established nitrates —typically 3— weeks). Then add fish gradually, a few at a time, so the biological filter can adjust to the increasing bioload.
Sources
- Aquarium Forge Stocking Algorithm v1 —editorial guidelines
- Species data reviewed against widely accepted hobby care standards
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