Aquarium Stocking Calculator & Fish Compatibility Planner

Use our how many fish in aquarium calculator, which uses a bioload scoring system rather than the simplistic inch-per-gallon rule. Each species is assigned a bioload score based on body shape, adult size, diet, and waste output. The target is ≤100% of the tank's calculated bioload capacity, with a warning threshold at 80%. A 60-litre tank can safely hold a bioload score of up to 60 points — for example, 8 ember tetras (0.5 pt each) plus 6 pygmy corydoras (1 pt each) = 10 points total, well within capacity.

No — the 1 inch per gallon rule is a rough and often misleading heuristic developed for small community fish in the 1950s. It fails for wide-bodied fish (goldfish, cichlids), tall fish (discus, angelfish), and high-waste species. Modern fishkeeping uses bioload scoring, surface area, and filtration turnover rate as more accurate stocking metrics. Our fish tank gallon calculator uses per-species bioload coefficients for 125+ species.

Bioload capacity is primarily determined by the nitrogen cycle efficiency of your filtration and the dissolved oxygen levels at your water's surface. As a guideline, our calculator assigns a capacity of 1 bioload point per litre of net water volume. Each species entry has a pre-set bioload coefficient (light, moderate, or heavy). Heavy bioload fish like goldfish or large cichlids rate 3–5× higher than similar-sized tetras.

Yes. Live plants consume ammonia and nitrate directly as plant fertiliser through the nitrogen cycle, reducing the biochemical oxygen demand on your filter. A heavily planted tank (>50% plant coverage) can support approximately 20–30% more bioload than an equivalent unplanted tank. Our calculator applies a plant bonus modifier to bioload capacity when you select plants in the Plant Planner tab.

Our compatibility checker evaluates three parameters for every fish combination in your stocking list: (1) temperature overlap — all species' minimum–maximum ranges must intersect; (2) pH overlap — all pH ranges must share a common value; (3) aggression flags — species tagged as fin-nippers, territory-holders, or predators trigger warnings against vulnerable tank-mates. Conflicts are highlighted in the Biological Parameter Check panel with specific species callouts.

The number of fish depends on your tank volume, species bioload, and filtration capacity. The old '1 inch per gallon' rule is inaccurate. Use a metabolic bioload calculator that factors in body mass, metabolic rate, and waste output. A 20-gallon tank can hold 10–15 small tetras or 2–3 medium cichlids, depending on species.

Bioload is the total biological waste produced by all fish and invertebrates in your tank. It includes ammonia from respiration and uneaten food. Higher bioload requires more filtration and more frequent water changes. Bioload scales exponentially with fish size — a 6-inch Oscar produces roughly 20x the waste of a 1-inch Neon Tetra.

Check three key parameters: temperature range overlap, pH range overlap, and aggression compatibility. Schooling fish need minimum group sizes (typically 6+). Avoid housing large predatory fish with small nano species. Use an aquarium fish compatibility checker that screens for temperature, pH, and predation conflicts automatically.

Aim for 60–80% bioload capacity for a balanced, low-maintenance tank. Going above 80% increases ammonia spikes and requires more water changes. Below 40% is very conservative and ideal for beginners. The aquarium stocking calculator shows your real-time bioload percentage based on the species you select.

Yes. Live plants absorb ammonia, nitrites, and nitrates, acting as secondary biological filtration. Fast-growing stem plants like Rotala and Hygrophila can increase your effective stocking capacity by up to 50%. Add plants from the CO2 calculator to see the bioload bonus reflected in the stocking calculator.