Tank Stocking Calculator

Last updated: November 2026

Enter your tank's footprint and depth, optionally pick a species, and get an honest stocking range from two classic methods — plus hard species minimums that no rule can talk you out of.

What the numbers mean

The surface-area method estimates oxygen exchange: gas swaps only at the water's surface, so footprint in square feet is a better oxygen budget than raw volume. We allow roughly 12 inches of small adult fish per square foot of surface — generous for a well-filtered, aerated tank, and a hard ceiling for a bare-bones one. The volume rule uses 1 inch of adult fish per 2 gallons of net water (half the old one-inch-per-gallon myth, which assumes perfect filtration that no one actually has).

The final estimate is the lower of the two, because oxygen exchange and bioload are both real limits, and the tighter one governs. Species minimums (5 gal for a betta, 20 gal for one common goldfish, 29 for angelfish) apply on top: a "safe number" in a sub-minimum tank is still a no.

Every stocking rule is rough — these are guardrails, not law

Actual capacity depends on filtration turnover, planting, temperature (warm water holds less oxygen), species temperament, and how diligently you do 25–40% weekly water changes. Understocking beats overstocking every single time: overstocking is the root of most chronic ammonia, oxygen, and disease problems this site exists to treat. Severe cases: consult an aquatic-experienced vet where available.

How many fish can I put in a 20 gallon tank?

The most common version of this question, answered for the standard 20-gallon long (30 × 12 in): a well-filtered 20 will hold roughly 12–15 small community fish (a school of 10 neon tetras plus 6 corydoras is about the honest ceiling — pick one school, not both), one betta solo, a guppy trio-plus with plants, or two fancy goldfish (one common goldfish is the minimum-sized resident here). Two young angelfish push past a 20's honest capacity — that species wants 29+ tall. Run your exact footprint through the calculator above for the surface-area and volume numbers behind these calls.

Quick presets, hard numbers

Species	Minimum tank	Adult size	Notes
Betta	5 gal heated	2.5–3 in	One per tank; no males together, never with fin-nippers.
Common goldfish	20 gal (first), +10 each	10–12 in	Production fish, not bowl fish.
Fancy goldfish	10–20 gal	6–8 in	Swim bladder prone; needs food soaked and greens.
Neon tetra	10 gal	1.5 in	Groups of 6+ or they hide and stress-clamp.
Guppy	5–10 gal	1.5–2.5 in	1 male per 2–3 females or population explodes.
Corydoras	10–20 gal	2–2.5 in	Groups of 6+, smooth substrate only.
Angelfish	29 gal tall	6 in+ across fins	Eats anything that fits in its mouth.