🐟 Fish Stocking Calculator
Enter your tank volume and each species you keep to see whether you're understocked, well stocked, or overstocked — a quick community check built on the inch-per-gallon rule.
🧮 Check Your Stocking Level
What is a Fish Stocking Calculator?
It estimates how full your aquarium is by weighing the combined adult length of your fish against the tank's volume. Add each species with its grown-up length and how many you keep, and it totals the inches, works out your stocking percentage, and tells you whether the tank is understocked, well stocked, or overstocked.
The inch-per-gallon rule is a rough guide rather than an exact science — filtration, bioload, body mass, territory, and compatibility all matter — so treat the result as a sanity check. It's a fast way to catch obvious overstocking before you add that next batch of fish.
❓ Frequently Asked Questions
How does the inch-per-gallon stocking rule work?
The rule of thumb allows roughly one inch of adult fish length per US gallon of water. Add up the full grown-up length of every fish (length × number of that species) and compare it to your tank volume. Under 75% of capacity is understocked, 75–100% is well stocked, and over 100% is overstocked. This calculator does the sum for a mixed community for you.
Is the one-inch-per-gallon rule accurate?
It's a useful starting sanity check, not a hard law. It ignores fish body mass — a chunky goldfish produces far more waste than a slender tetra of the same length — and it doesn't account for filtration, territory, or activity level. Use it to spot obvious overstocking, then refine with species-specific research on bioload and compatibility.
Should I use adult length or the size at the store?
Always use the full adult length, not the small juvenile size you buy. Many common species grow far larger than they appear in the shop tank, so stocking to their bought size leads to a badly overstocked tank within months. Look up the maximum adult length for each species and enter that.
What happens if my tank is overstocked?
Overstocking overloads the biological filter, so ammonia and nitrite can spike and nitrate climbs quickly between water changes. That stresses fish, stunts growth, fuels aggression over territory, and invites disease. If you're over 100%, upgrade filtration and increase water-change frequency, or rehome some fish or move up to a larger tank.