Who makes this
My name is Joseph Lawrence. I build small, single-purpose tools for decisions where the arithmetic is annoying and most of the people offering to help you have a stake in the answer.
What I am not
I am not an ichthyologist, a veterinarian or a professional aquarist. I keep tanks, I read the literature, and I have made most of the mistakes this tool warns about — which is a different thing from being an authority, and I am not going to imply otherwise.
What I do instead is show the work. The bioload model, the four capacity limits and the seven compatibility checks are all published on the calculator page, with the constants stated. If you disagree with a number you can see exactly which number to disagree with.
Where the species data comes from
Each entry is compiled from general aquarium literature and long-standing keeper consensus, cross-checked against multiple independent sources before it goes in. Ranges are given as ranges, because that is how the data honestly exists — no serious source will tell you a cardinal tetra needs exactly pH 6.4.
Adult sizes are realistic captive adult sizes rather than maximum recorded lengths, because the question you are asking is what will be in your tank in two years, not what the record holder managed in a public aquarium.
Species I am not confident about are left out. A shallow database that guesses is worse than a smaller one that does not, particularly in a hobby where people will notice and say so.
On being argued with
Aquarium compatibility is contested and it always has been. Whether serpae tetras can be trusted in a community, whether a betta belongs with anything at all, whether an oscar needs 75 gallons or 125 — experienced keepers disagree, publicly and at length, and most of them have the tanks to back up their view.
The approach here is to be conservative and to say when it is contested. Where the hobby splits, the species note records the split rather than pretending to certainty. Where a pairing is borderline, the matrix shows a caution rather than a tick. You will still disagree with some of it. That is fine — the reasoning is on the page, so you can disagree with something specific.
How this is paid for
Display advertising, and nothing else. I do not sell fish, plants, tanks or equipment, take affiliate commission from anyone who does, or accept payment for a species to be recommended. If that ever changes it will be disclosed on the page it affects, and it will never sit on the advice a tool gives.
Corrections
If something here is wrong, tell me and I will fix it. Corrections with a source are the most useful message you can send. Contact.