Aquarium stocking calculator
Estimate how full your freshwater tank is using adult size and bioload for each species, rather than the inch per gallon rule.
Fish
How the estimate works
Each species in the library carries an adult length, a bioload factor, a minimum tank size and a temperature range. The bioload factor stands in for how much waste a fish of that type adds relative to a small tetra, so a messy four inch fish counts for far more than its length alone. The tool adds up the bioload of every fish you enter.
Capacity is based on your net volume, then adjusted by filter turnover. A filter that moves the water around five times per hour supports the baseline capacity, while stronger or weaker flow nudges it up or down within sensible limits. Percent stocked is the total bioload divided by that adjusted capacity. Below roughly seventy percent reads as understocked, seventy to one hundred as well stocked, and above one hundred as overstocked.
Reading the warnings
Percent stocked only covers waste load and rough space. It cannot see whether a species will outgrow your footprint, so the tool checks every species you enter against its minimum tank size and flags any that need more water than you have. A school of active swimmers can be under the stocking limit yet still be cramped in a short tank, which is why the minimum size warning is separate from the percentage.
Use the result to plan, then research temperament, schooling numbers and water parameters before you buy. Add fish gradually so your filter bacteria can keep pace, and retest your water in the weeks after any new addition.
Frequently asked questions
Why not just use the one inch per gallon rule?
The inch per gallon rule ignores body mass, waste output and swimming needs. A single four inch fish produces far more waste than four one inch fish, so this tool uses adult size and a bioload factor per species instead.
What does percent stocked mean here?
It compares the total bioload of your fish against an estimated capacity for your volume, adjusted for how much your filter turns the water over. Around 70 to 100 percent is a comfortable, well stocked range.
Can I trust the result for aggressive or territorial fish?
Treat it as a starting point. The number covers waste load and space, but behavior, territory and compatibility still need research before you combine species.
Why does it warn about minimum tank size?
Some species need a minimum footprint or swimming length regardless of bioload. If a species minimum is larger than your tank, the tool flags it even when the percent stocked looks fine.