How Many Fish Can My Tank Hold?
Overstocking is one of the most common mistakes beginner aquarists make. Too many fish leads to spikes in ammonia and nitrite, increased disease, stressed fish, and a tank that's much harder to maintain. This calculator helps you avoid that by checking two widely-used stocking methods at once.
The Legacy Inch-Per-Gallon Rule (and Its Limits)
The simplest way to estimate stocking: allow 1 inch of adult fish body length per US gallon of water. A 20-gallon tank could hold 20 inches worth of fish — that might be 10 neon tetras at 1.5 inches each, or 4 platies at 2.5 inches each. We show it only as a rough cross-check: it treats a slim tetra and a deep-bodied goldfish as equivalent, and ignores filtration performance and swimming terrain entirely — the factors that actually decide whether a stocking plan is healthy.
Swimming Terrain: The Surface Area Method
A more accurate approach: allow 1 inch of fish per 12 square inches of surface area. This is because oxygen exchange happens at the water surface — more surface area means more oxygen, which means more fish can be supported. A 24 × 12 inch tank has 288 in² of surface area, allowing up to 24 inches of fish. This method rewards tanks with larger footprints over taller tanks.
Bioload Capacity & Filtration Performance
A heavily planted tank with lots of live plants naturally processes more waste and produces more oxygen, allowing you to stock slightly more fish. A predator or aggressive tank, on the other hand, requires more space per fish to reduce territorial conflict and stress — so we apply a 30% reduction to the maximum capacity.