Aquarium Fish Compatibility Checker

Building a successful community aquarium requires matching fish across several compatibility dimensions. This tool scores the two you can put numbers on: temperament and water parameter overlap (pH and temperature).

Space is the third big factor and it is not scored here, so check each species’ minimum tank size separately. Fin shape is a fourth. A betta and an angelfish score perfectly on every number below and still tend to shred each other’s fins.

Temperament Scoring: Each fish species is rated on an aggression scale from 1 (peaceful) to 5 (highly aggressive). Compatible pairings should have an aggression difference of no more than 1 point. For example, a Neon Tetra (aggression = 1) pairs well with a Corydoras Catfish (aggression = 1) but poorly with an African Cichlid (aggression = 4).

Compatibility Score = 100 - (Aggression Difference × 20) - pH Penalty - Temp Penalty

Water Parameter Overlap: You keep one tank at one pH and one temperature, so what matters is not how far apart two species’ preferences sit. It is whether their tolerated ranges share a window you can actually hold. Two fish listed at pH 6.5 and 7.2 sound half a point apart, but if one tolerates 6.0 to 7.0 and the other 6.8 to 7.8, there is a real shared band at 6.8 to 7.0 and the pairing works.

That is why this calculator asks for each species’ low and high figure rather than a single number. Look them up on the species profile from your supplier, or use the reference table below.

pH penalty:

• Shared window of 0.4 pH or wider: no penalty
• Shared window narrower than 0.4: (0.4 - window) × 25
• No shared window, gap of g: 10 + g × 40

Temperature penalty (°F):

• Shared window of 4°F or wider: no penalty
• Shared window narrower than 4°F: (4 - window) × 2.5
• No shared window, gap of g: 10 + g × 5

A narrow window is not a hard fail, it is a maintenance burden. A tank with 0.2 pH of headroom drifts out of range every time you skip a water change.

Worked Example: Fish A: Neon Tetra (aggression 1, pH 6.0 to 7.0, temp 72 to 78°F). Fish B: Guppy (aggression 1, pH 6.8 to 7.8, temp 72 to 82°F).

• Aggression difference: 0 → 0 penalty
• pH windows overlap at 6.8 to 7.0, a window of 0.2 → (0.4 - 0.2) × 25 = 5 penalty
• Temperature windows overlap at 72 to 78, a window of 6°F → no penalty
• Score = 100 - 0 - 5 - 0 = 95 (Excellent)
• Hold the tank at pH 6.9 and 75°F, the middle of both shared windows

Reference: Common Fish Profiles:

Scores above 80 indicate excellent compatibility. Scores between 50 and 80 mean monitor closely. Below 50 means the pairing is risky and likely to cause stress or aggression.