How we rate compatibility

TankMates rates 72 popular freshwater species (2,556 pairs) with explicit, tested rules. Every verdict shows its reasons so you can judge for yourself.

The data

• Temperature and pH: the aquarium range published by Seriously Fish, cross-checked against FishBase. Where they disagree by more than 2 °C (or 0.5 pH), we review by hand and note the decision on the species page.
• Shrimp, snails, goldfish and the common pleco, which those references don't cover well, use at least two other published care guides (Wikipedia, Aquarium Co-op, Aqueon, Fishkeeping World). These are listed on each page.
• Adult size and minimum tank: the larger of the reference value and our own record, so our figures err on the generous side for the fish.
• Behaviour flags (temperament, fin-nipping, territorial, predatory, shrimp/snail eating, schooling): our assessment based on the same references and widely reported hobby experience.
• 152 source pages are cited across the database. Data last reviewed 2026-11-04. A monthly automated check re-reads the source pages and flags changes for review.

The rules (worst result wins)

• Temperature: shared range under 1 °C → not recommended; under 2 °C → caution. Coldwater + tropical fish with under 2 °C overlap → not recommended.
• pH: no overlap → caution (captive-bred fish often adapt).
• Predation: active predators at 2.5× the other's length (opportunistic eaters at 4×) → not recommended; at 1.8× (2.5×) → caution.
• Fins: known fin-nippers with long-finned fish → not recommended; occasional nippers → caution. Bettas and paradise fish with long-finned fish → not recommended.
• Aggression: aggressive with peaceful → not recommended; much larger territorial fish → caution; two territorial species that use the same part of the tank → not recommended.
• Shrimp and snails: fish that eat adult shrimp or snails → not recommended; fish that eat baby shrimp → caution for breeding colonies.
• Stocking: one slim 5 cm fish = 1 unit; units scale with (length ÷ 5 cm)^1.5 × body-mass factor; about 1 unit per US gallon is full.

The rules are checked against 50+ textbook pairings (for example betta + guppy = not recommended, discus + cardinal tetra = compatible) before every update. If a data change flips one of them, the site doesn't publish.

Limits

Fish are individuals. A "compatible" pair can still fight, and some "caution" pairs work well in large, planted tanks. Tank size, decor, sex ratio and the order you add fish all matter. Freshwater only; no brackish or marine species.

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