How the stocking sums work
Each verdict answers one question, and none is averaged into another. Here is what each one measures.
Space
Can each fish turn round and swim? The longest side of the tank is compared with the length each species needs, which is a multiple of its adult size or its stated minimum, whichever is larger. A short tank fails here whatever its volume.
Load
How much waste the stock produces, against the water available to dilute and filter it. Every species has a bioload weight. Weights are summed, softened by an exponent so a big fish is not counted as a hundred small ones, and compared with usable water, which is the brim volume less 15 per cent for substrate, décor and the gap below the rim.
Water
Do the fish share a temperature range, a pH range, a hardness range and, for marine and brackish fish, a salinity? A shared range of less than a couple of degrees reads Tight, and no overlap reads Over. Fish from soft acid blackwater and fish from hard alkaline lakes do not belong in the same tank however roomy it is.
Group
Shoaling fish need a minimum number, and some species need a particular ratio of males to females. Territorial fish that cannot be kept together, fin-nippers and fish that would eat their neighbours are flagged here. A short list of known pairs, held in a separate table, catches combinations the general rules would miss.
What the numbers are not
They are planning figures from a data table of 1,000 species, with the estimated fields marked as estimates. They do not know your filtration, your plants or how well you feed. Treat Fits as a good start and Over as a real warning. The method page lists the sources of the data and the known gaps.