Compatibility Checker
"Will these two get along" is only part of what compatibility means, and it's often the part people focus on while the parameters that actually sink a stocking plan — mismatched adult size, no water-parameter overlap, or two fish competing for the same water-column niche — go unchecked. This tool weighs temperament alongside water-parameter overlap, adult size difference, which part of the tank each species occupies (surface, mid-water, or bottom), and whether either species needs to be kept in a group rather than singly or in a pair.
How to use it: add the species you're considering, or already keep, to the list. Each pairing gets a color rating — green, amber, or red — along with the specific reason behind that rating, so you're not just told "no," you're told what would need to change (a bigger tank, a larger school, different tankmates) to make it work.
This draws on the same species profiles used throughout the site, covering 268 species' typical temperament, adult size, water parameter ranges, and social needs.
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What Green, Amber, and Red Mean
Green means the pairing overlaps comfortably on all the major factors — parameters, size, niche, and temperament — with no significant caveats for a typically-sized community tank. Amber means the pairing can work but comes with a real condition attached: enough swimming space, a large enough school size, a tank big enough that size difference doesn't matter, or careful stocking density to avoid resource competition. Red means the pairing carries a substantial risk of injury, stress, or death under most home aquarium setups — not impossible in every conceivable circumstance, but not a pairing to build a stocking plan around.
Why "Gets Along" Isn't the Whole Story
Two fish can be entirely peaceful toward each other and still be a poor pairing. A peaceful giant gourami and a peaceful neon tetra aren't a good match because the gourami will eventually see the tetra as food, not because of aggression — that's a size mismatch, not a temperament problem. Similarly, two fish with compatible temperaments but non-overlapping parameter needs (a soft-water Amazonian species paired with a hard-water rift lake cichlid) will be chronically stressed by whatever compromise water chemistry you settle on, even without a single act of aggression between them.
Worked Examples
Betta and a school of six or more neon tetras in a tank of adequate size typically rates green-to-amber: most bettas tolerate fast-moving mid-water schoolers reasonably well, particularly with a large enough school that no single tetra gets singled out, though individual betta temperament varies enough that this isn't a guarantee.
Two male bettas in the same tank rates red without exception in a standard setup — male bettas are highly territorial toward other male bettas specifically, and this isn't a "sometimes" risk, it's close to universal.
Goldfish and a tropical community fish like a neon tetra rates red on parameter grounds alone: goldfish are cool-water fish with substantial bioload, and the temperature and water-quality compromise needed to keep both comfortable rarely favors either species.
A cichlid several times the size of a small tetra, even a generally peaceful cichlid species, tends to rate amber-to-red on size grounds — a mouth that can physically fit a tankmate is a real risk regardless of stated temperament, since predation isn't really an aggression issue, it's a size-ratio issue.
Frequently Asked Questions
What if a pairing shows amber — is that a yes or a no?
Amber means workable under specific conditions, which the result explains — usually tank size, school size, or stocking density. Read the specific condition rather than treating amber as a blanket approval or rejection.
Does the checker account for individual fish temperament?
No, and this is a real limit worth knowing — ratings reflect typical species behavior, but individual fish vary, sometimes substantially. A red-rated species pairing that appears to work in one specific tank doesn't mean the underlying risk wasn't there; a green-rated pairing occasionally still produces one unusually aggressive individual.
Why does tank size change the rating for some pairings?
Because several conflicts are about space, not species. A pairing that's amber in a 20-gallon tank can shift to green in a 75-gallon tank simply because there's enough room for both species to establish separate territory or swimming space, reducing forced competition.
Can I check more than two species at once?
Yes — add your full planned stocking list and the tool checks every pairing, since a combination that's fine two species at a time can still create problems once a third species is added and competing for the same niche.
Does this replace checking water parameter overlap separately?
No, it's built into the compatibility rating itself, but it's still worth reviewing your specific tap water parameters using the water parameter guide, since a species pairing can overlap on paper while your actual local water sits outside a comfortable range for one of them.