Aquarium Stocking Calculator
Calculate your fish tank stocking level using the inch-per-gallon rule. Add multiple fish species, see capacity percentages, and avoid overstocking your aquarium.
About This Tool
Maintaining a healthy aquarium requires balancing the number and size of fish with the capacity of your tank. Overstocking is one of the most common mistakes new fishkeepers make, and the consequences are serious: elevated ammonia and nitrite levels, oxygen depletion, increased disease transmission, and stressed fish with shortened lifespans. This Aquarium Stocking Calculator helps you avoid those problems by applying the widely used inch-per-gallon guideline.
The inch-per-gallon rule is a simple starting point for freshwater tanks: for every gallon of water, you can support approximately one inch of fish at its adult size. Saltwater tanks require significantly more space per fish, typically 2 to 3 gallons per inch, due to the more demanding biological load and the sensitivity of marine species. This calculator automatically applies the correct ratio based on your tank type.
You can build your fish list by adding species with their adult size and quantity. The calculator then totals up the inches of fish, compares them against your tank's capacity, and reports a stocking percentage along with a status indicator. Understocked tanks under 85 percent have plenty of room. Well-stocked tanks between 85 and 100 percent are at a comfortable level. Overstocked tanks above 100 percent need immediate attention, and dangerously overstocked tanks above 120 percent pose a severe health risk to all inhabitants.
The remaining capacity display helps you plan future additions. If you have 4 inches of remaining capacity, you know exactly how many small tetras or how much of a larger fish you can safely add. Always use the adult size of the fish, not the current juvenile size, since most aquarium fish are sold young and will grow significantly.
Key Features
- Applies the inch-per-gallon rule with automatic adjustment for freshwater and saltwater tanks
- Dynamic fish list lets you add, edit, and remove species with name, size, and count fields
- Color-coded stocking status from understocked to dangerously overstocked for at-a-glance assessment
- Shows remaining capacity in inches so you can plan future fish additions precisely
- Per-fish breakdown displays each species' contribution as a percentage of total capacity
Frequently Asked Questions
Is the inch-per-gallon rule always accurate?
It is a useful starting guideline but has limitations. The rule works best for small, slim-bodied fish like tetras and danios. Larger, bulkier fish like goldfish or cichlids produce more waste per inch and need 2 to 3 gallons per inch even in freshwater. Active swimmers also need more space regardless of body size. Use this calculator as a baseline and research species-specific requirements.
Should I use adult size or current size when calculating?
Always use the expected adult size. Most fish sold at pet stores are juveniles and will grow substantially. A common pleco, for example, is often sold at 2 inches but can reach 18 to 24 inches as an adult. Planning based on current size leads to severe overstocking as your fish mature.
What can I do if my tank is overstocked?
Increase filtration capacity to handle the extra bioload, perform more frequent water changes (twice weekly instead of weekly), add live plants to help absorb ammonia and nitrates, and monitor water parameters closely with a test kit. Long term, consider rehoming some fish or upgrading to a larger tank. Chronic overstocking shortens fish lifespans significantly.