Where these disagree with somebody
Fish compatibility is genuinely contested, and these guides say so rather than pretending otherwise. Two experienced keepers will give you opposite answers on serpae tetras, on whether a betta belongs in a community tank, on how many gallons an oscar needs and on how long quarantine should run — and both will have kept the fish for years.
Where the hobby splits, the guide names the split and explains what each side is weighing. Where a figure is a range rather than a number, it is written as a range, because that is how the data honestly exists.
To apply all of it to a specific tank at once, use the stocking calculator — it checks volume, filtration, surface area and footprint together and screens every pair of species you pick against each other.
General guidance compiled from standard aquarium literature and long-standing keeper practice, using the species database published on this site. Figures are ranges where ranges are what exist. Test kit readings from your own tank always take precedence. Not veterinary advice.