Aquarium Stocking Calculator

How the checker works

Most stocking calculators give you one percentage and let a bigger filter push it up. That hides the problems that actually kill fish: a tank that is too short for an active schooling fish, a predator that eats its tank mates, or two species that need different water. So the checker looks at each of those separately, and tells you why and what to change.

1. Room. Each species has a minimum tank from its care source, usually a footprint such as 60 × 30 cm. If your tank is shorter or smaller, you get a Check. If it is clearly too small (shorter than 4 × the fish's adult length, or less than half the recommended size), you get a Problem.
2. Group size. Schooling fish kept in small numbers are stressed and often nip or hide. Species that fight their own kind (male bettas, red-tailed and rainbow sharks) are flagged if you add more than one.
3. Water. The checker finds the temperature and pH range that every species tolerates. If there is none, they can't share a tank. Where sources disagree, we keep both: the main care source's range, and the narrower range the sources agree on.
4. Tank mates. Predators are flagged with fish under a third of their adult length (or a known prey size), fin-nippers with long-finned fish, and aggressive species with peaceful ones. Shrimp, snails and live plants are checked against each fish.
5. Load. A rough guide to waste, shown as Light, Moderate, Heavy or Overstocked rather than a falsely exact percentage. See below.

How the load is estimated

Waste depends on how much a fish eats, which depends on its body mass, not its length. A 15 cm angelfish weighs far more than five 3 cm neons. So the checker estimates each fish's adult mass from its length, then scales it by metabolic rate, which in fish rises a little slower than mass:

Fish of normal shape grow roughly by the cube of their length (the "cube law"; see Froese 2006), and 0.01 × L³ is a typical-shape estimate: deep-bodied fish such as discus weigh more, slim ones less. The 0.8 exponent is from Clarke & Johnston (1999), who compared 138 studies of 69 fish species. The band limits are our own estimates, set so that the worked examples below land where experienced keepers would put them. Real water means the water left after glass, the gap below the rim and the substrate, as on our tank size guides.

Example 1: 20 gallon long community

A 20 gallon long (30 × 12 × 12 in) with live plants: 10 neon tetras, 6 panda corydoras and 6 otocinclus.

• Real water: with 3/16 in glass, a 1 in gap and 2 in of sand, the water is 29.625 × 11.625 × 8.8125 in = 3,035 in³ = 49.7 L (13.1 gal).
• Room: neons need a tank at least 60 cm (24 in) long and 54 L; the 20 long is 30 in and 71 L, so it passes. Corydoras and otos need less.
• Groups: 10 neons (8+), 6 pandas (6+), 6 otos (6+): all fine.
• Water: pandas set the upper limit at 25 °C, so the shared range is 22–25 °C (72–77 °F), with pH 6.0–7.0 ideal for all three.
• Load: a 3 cm neon is about 0.27 g, giving 0.35 load units; a 5 cm panda 1.20; a 3.5 cm oto 0.51. Total 10 × 0.35 + 6 × 1.20 + 6 × 0.51 = 13.7 ÷ 49.7 L = 0.28: Moderate.

Verdict: Looks good. There's room for a few more small fish, but not for a single large one.

Example 2: betta, shrimp and a snail in a 10 gallon

A 10 gallon (20 × 10 × 12 in), planted: 1 betta, 10 cherry shrimp and 1 nerite snail.

• Real water: 19.625 × 9.625 × 8.8125 in = 1,665 in³ = 27.3 L (7.2 gal).
• Room: SeriouslyFish suggests a 45 × 30 cm base for a betta, about 41 L. A 10 gallon is 51 cm long but holds 39 L by its outside size, just under, so you get a Check, not a Problem.
• Tank mates: bettas may eat shrimp, especially babies, so the shrimp get a Check: plant the tank densely. The snail is safe.
• Water: the nerite needs harder water (pH 7.2 or more), so the shared range is pH 7.2–8.0 at 24–26 °C (75–79 °F).
• Load: betta 1.85 + 10 shrimp × 0.35 + snail 0.23 = 5.6 ÷ 27.3 L = 0.21: Moderate, just over Light.

Verdict: Check 2 things. A workable tank, but expect to lose some baby shrimp.

Example 3: angelfish with neon tetras in a 55 gallon

A 55 gallon (48 × 13 × 21 in): 4 angelfish, 12 neon tetras and 6 bronze corydoras.

• Real water: with 5/16 in glass, 47.375 × 12.375 × 17.6875 in = 10,370 in³ = 169.9 L (44.9 gal).
• Tank mates: adult angelfish eat small fish, and care sheets such as Fishkeeper's warn specifically against neon tetras. The checker treats fish of about 4 cm or less as prey for an adult angel (our estimate from those warnings). A 3 cm neon is food: Problem.
• Load: a 15 cm angelfish is about 34 g, or 16.7 load units, as much as 48 neons. 4 × 16.7 + 12 × 0.35 + 6 × 3.16 = 90.0 ÷ 169.9 L = 0.53: Heavy.
• The fix: swap the neons for 12 rummy-nose tetras (5 cm adults, too big for the angels to swallow). The problem disappears; you are left with a Check to keep an eye on them, and the heavy load.

Verdict: Not recommended as entered. This classic mix often works while the angels are small, then the neons start disappearing.

Where the species data comes from

Every species was researched against its care profile at SeriouslyFish and its entry at FishBase, with Planet Catfish, Practical Fishkeeping and other specialist sources where those had gaps. Each species in your results links to the pages used. Where sources disagree we chose the safer figure for the fish (the bigger tank, the bigger group) and show you both temperature ranges. Species marked medium confidence had fewer sources or larger disagreements.

Two choices are deliberately strict. Goldfish get their own large minimums, because they grow far bigger and messier than most people expect. And the "Siamese algae eater" sold in shops is usually Crossocheilus langei rather than true C. oblongus; the data follows the fish you can actually buy.

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