The prettiest fish in the shop can be the worst neighbour in your tank. Some fish nip fins, some eat smaller tank mates, and some need water so different that one species always suffers. This free planner checks two species against each other on temperature, pH, and temperament before you buy, so you can shortlist combinations that have a real chance of living together in peace.
How to use this planner
Pick a first species and a second species from the lists, then press check compatibility. The tool compares their temperature ranges and pH ranges for overlap, flags aggressive or semi-aggressive temperaments, and warns when a betta or angelfish may eat cherry shrimp. A clean result means the pair looks compatible on those three checks, not that every individual fish will behave, since personality, tank size, and hiding places still matter.
What the checks cover
Temperature overlap matters because a fish kept too cold eats poorly and falls ill, while one kept too warm ages fast and runs out of oxygen. pH overlap matters less than stability, but a wide gap means one species always lives at the edge of its comfort zone. Temperament matters most day to day: aggressive fish chase and injure tank mates, semi-aggressive fish may bully peaceful ones, and even peaceful fish can turn territorial when breeding or crowded.
A worked example
Neon tetras and guppies pass all three checks, since both like warm water in the low to high twenties and peaceful company, though their pH sweet spots differ slightly. A male betta with guppies fails the temperament check, since bettas often attack bright, flowing fins. An angelfish with cherry shrimp fails the predation warning, since adult angelfish eat small shrimp. A molly with a neon tetra flags a pH gap, since mollies prefer harder, more alkaline water.
Reading your result
A clean result is a green light to research further, not a final verdict. Check adult size, since a peaceful juvenile can outgrow and eat tank mates later. Check tank size and group size, since schooling fish need groups and territory needs space. A caution result means think twice, add more hiding places, or pick a different species. Two cautions, or a predation warning, usually means choose another combination rather than hoping for a gentle individual.
Tips for peaceful tanks
Add the most peaceful species first and the boldest last, so territories form gently. Keep schooling fish in proper groups of six or more, since lonely schooling fish stress and nip. Give every species a place to retreat: plants, wood, and caves break sight lines and end chases. Feed a varied diet so hunger does not turn tank mates into food. Quarantine new fish for a few weeks, since a sick newcomer can harm a whole peaceful tank.
Common mistakes
The classic mistake is buying on colour alone and discovering aggression at home. The second is trusting a shop label that says peaceful for a fish that turns territorial as an adult. The third is overcrowding, which turns even calm species nippy. The fourth is mixing fish that need opposite water, then chasing pH with chemicals, which stresses every fish in the tank. Plan the community on paper first, with this tool, and the tank will stay calmer.
Frequently asked questions
Can two male bettas live together? No, they fight, often to the death, so keep one male betta per tank. Do guppies and tetras mix well? Usually yes, in warm, planted tanks with gentle flow, though very fancy guppy fins can tempt nippers. Can shrimp live with any fish? Small shrimp are safest with small peaceful fish and lots of plants, and risky with bettas, angelfish, and any large fish. What if my fish still fight? Add hiding places, check crowding, and be ready to separate them.
A note on limits
This planner covers eight common species and three checks, so it cannot cover every fish, every hybrid, or every individual personality. Always research the adult size, diet, and breeding behaviour of each species before buying, and ask keepers who have run the exact combination. Compatibility is a starting screen, not a guarantee, and a well-planted, well-sized, lightly stocked tank forgives far more than a bare, crowded one.
Versión en español: Compatibilidad de peces de acuario – Qué especies pueden convivir.