How many fish per gallon?

The inch rule's failures, the bioload-points replacement, and worked stocking lists you can copy.

Where 1-inch-per-gallon breaks

Direction one: a 10-inch oscar "fits" a 10-gallon by the rule and dies in it — biologically impossible, chemically instant ammonia, physically no room to turn. Direction two: twelve 1-inch neons in 10 gallons is a half-stocked tank by the rule, but it's actually a nicely shoaling, properly filtered setup. Length isn't load; mass is, and behavior (territorial cichlids vs. shoaling tetras) is a second axis the rule ignores entirely.

Bioload points — the working system

Capacity ≈ 1 point per gallon for a normally filtered community tank with weekly changes. Messy-fish setups (goldfish, plecos, big cichlids) run 0.6 points per gallon — count their class up one notch and be strict about changes. Planted tanks with fast stem plants buy 10–20% headroom, not a second tank's worth.

Worked stocking lists

10 gallon (10 pts)

8 neon tetras + 3 pygmy corys (8+3×1=11, run 90% and watch nitrates) — or 6 guppies + 1 honey gourami (6+3=9). One betta alone is a full 10-gallon stock by temperament, not points.

20 gallon long (20 pts)

10 neons + 4 corys + 1 honey gourami = 10+4+3 = 17 pts (85%, comfortable). Or a single-fish show: 1 fancy goldfish (heavy class: count 14 pts against 12 pts capacity — a 30 gal is the honest home).

55 gallon (55 pts)

12 tiger barbs (12) + 8 corys (8) + 2 keyhole cichlids (6) + 1 bristlenose (8, messy-adjusted) = 34 pts — room to grow. This is how you buy margin: stock to 80%, watch nitrate slope for a month, then decide.

The three rules that actually keep fish alive

1. Cycle first. Four to six weeks of ammonia dosing or seeded media before the first fish — an uncycled tank kills faster than any stocking error. 2. Adult size. Research the three-year fish, not the shelf fish. 3. Add slowly. A few fish per fortnight; the bacterial colony scales to feeding, not wishes.