How Our Aquarium Compatibility Engine Works
Every verdict on this site comes from the same engine: nine scored dimensions, combined so that the worst problem — not the average — decides the outcome. This page explains exactly how that works, and what the engine does not know.
The nine dimensions
1. Tank size capacity
Every species has a minimum tank volume it needs to behave normally. A fish that needs 100 gallons does not become content in 40 — it becomes stressed, and stressed fish get sick or aggressive. We score how comfortably your tank volume covers each species’ minimum.
2. Cumulative bioload
Each animal adds waste to the system, and your filtration has a budget. We track how much of the tank’s biological capacity is already spoken for by current residents, so the tenth fish is judged against a fuller tank than the first.
3. Reef safety
Some fish eat coral. Some only nip when underfed. We classify species as reef safe, safe with caution, or unsafe, and score against your tank type — a coral-nipper in a fish-only tank is fine; in a reef it is a problem.
4. Tankmate aggression balance
A peaceful goby and a dominant trigger have very different ideas about sharing space. We compare each candidate’s temperament against the most assertive current residents to catch bully-and-victim setups before they happen.
5. Swimming zone
Fish divide the tank vertically: some live at the surface, some hover midwater, some never leave the sand. Too many species claiming the same zone means constant friction even between "peaceful" fish.
6. Territorial overlap
Territorial species claim an area and defend it. Two territory-holders in a small tank cannot both win. We estimate territory demands against your tank’s footprint and flag overlap.
7. Diet compatibility
Predation is the bluntest incompatibility: if one resident fits in another’s mouth, eventually it will be in it. We check predator-prey size relationships and feeding styles — including fish that eat shrimp, snails, or coral polyps.
8. Social structure
Some species must school to feel safe; some pair; some tolerate their own kind only in harems; some will kill any conspecific on sight. We score whether the quantity you plan matches how the species actually organises itself.
9. Known pairwise conflicts
Some combinations fail for reasons no general rule captures — two Zebrasoma tangs, dottyback against dottyback, puffers with any invertebrate. We maintain a curated rule list for these, each with the tank-size threshold where the conflict eases, when one exists.
Why we don't average
Averaging lies. A fish that scores perfectly on six dimensions and lethally on one is lethal — a 7-gallon tank does not become acceptable for a tang because the diet and reef-safety scores are excellent. So the engine aggregates by weakest link: any dimension that scores critically low caps the overall score, and hard incompatibilities (predation, known fatal pairings) force a red verdict outright. A green verdict therefore means no dimension found a serious problem, not "the good outweighed the bad".
Where the pairwise rules come from
The curated conflict list encodes failure modes that aquarists rediscover the hard way every year, drawn from hobbyist literature, vendor husbandry guidance, and the accumulated record of public reef-keeping forums. Each rule names the species or genera involved, the reason, and — where the conflict is about space — the tank size above which it relaxes. Rules are deliberately conservative: they describe the typical outcome, not the lucky exception.
What the engine doesn't know
Individual temperament. Fish are individuals: there are murderous "peaceful" clownfish and gentle triggers, and no database can predict which one you will get. The engine also cannot see your rockwork, your feeding discipline, the order you introduce animals, or whether your "90 gallons" is tall and narrow. Treat a green verdict as "no known structural problem", not a guarantee — and treat a red verdict seriously, because those encode the failures that happen to most people, most of the time.
The data
The catalog covers over 1,000 marine species — fish, invertebrates, and corals — with structured care parameters: minimum tank size, temperament, reef safety, diet, swimming zone, schooling behaviour, and territoriality. Data is compiled from published husbandry sources and reviewed for consistency; where sources disagree we keep the more conservative value. When we are uncertain, the engine says so in its reasoning rather than hiding it.
Try it on your own stocking plan
The planner runs this engine on any tank you describe — no account needed.
Open the stocking planner →