How the planner decides

Every verdict is suitable, risky, not suitable, or not ready to stock, and lists one reason for each rule it checked. The worst finding decides the verdict. A guide, not a guarantee.

The rules

Cycling

A tank that has not finished cycling is not ready to stock, whatever is planned for it.

Minimum tank size

Each species' minimum volume and length. Below either is not suitable. Goldfish add volume for each additional fish.

Room to swim

Tank length against the adult size of each fish: under 3 times is not suitable, under 4 times is risky.

Bioload

The waste the whole plan makes against what the tank can process: from 80% it is risky, over 100% it is not suitable.

Shared temperature

The range every species tolerates. No overlap is not suitable; a window under 1 °C (1.8 °F) wide is risky.

Shared pH

The same, for pH. A window under 0.3 wide is risky.

Temperament

Predators against fish small enough to eat; fin nippers against long fins; fish that fight their own shape; shy fish against boisterous or much larger ones; fish against shrimp and snails; species that interbreed.

Group size

Schooling species below their group minimum are risky, below half of it not suitable. Species kept one or two to a tank are flagged above that.

Why not one inch of fish per gallon

That rule counts body length, so it treats one 10 inch fish the same as ten 1 inch fish. They are not the same: a fish's weight grows with the cube of its length, so the 10 inch fish weighs about a thousand times as much as one 1 inch fish, and makes far more waste than all ten together. This planner never uses it.

How bioload is worked out

Weight in grams is estimated as 0.012 × build × length³ (length in cm). Waste grows with weight to the power 0.75, the way metabolism scales in animals, and every animal adds a fixed 0.6 units for the room, oxygen and food it takes, so a tank of tiny fish still fills up. A cycled tank is taken to process 0.55 units of waste per litre of tank volume.

With those numbers, one 10 inch fish of average build makes 53 units and ten 1 inch fish make 9.

Where the facts come from

Each fact is read from named public sources, with the date it was read. Where sources differ, the site shows the most cautious value, computed from all of them: the narrowest temperature and pH range, the largest adult size, the largest minimum tank and the largest group. Sizes are the largest given, whether a source measures to the base of the tail (standard length) or to its tip (total length). All text is written for this site. See the sources.

Tank length

The planner starts from the usual length of a rectangular tank of the volume you enter, and uses yours if you change it.