Reef fish compatibility checker
List the fish you want and check them against each other, your tank size and the corals and shrimp you keep. Every warning comes with its reason, and you get an order to add them in.
Formulas and sources last checked . How we check
Calculator
Result
Add a fish to start.
How does the compatibility checker decide?
Every pair of fish is checked against the reasons fish fight, drawn from the species profiles we researched:
- the same species or genus, or a similar body shape, which trigger territorial fights;
- predator and prey, by the size of fish a predator can swallow;
- a peaceful fish with an aggressive one, which is a clash when the peaceful fish is small;
- two fish living on the same food, such as copepods, or the same sand or burrows;
- the smallest tank each fish needs, and known problem pairs from the research.
Each fish is also checked against the coral groups and shrimp you keep. The order puts peaceful and small fish first and territorial ones last.
Worked example: a first fish list
- Two ocellaris clownfish in 200 litres: a caution, because they should be a pair or two youngsters added together.
- Add a six line wrasse: fine with the clowns, but the planner suggests adding it after them, because it gets territorial.
- Add a mandarin dragonet: it eats copepods from the rock, and so does the six line wrasse. In a tank under about 400 litres that is a caution for food.
Try it: open this list in the planner.
How many fish can I keep?
There is no tested formula. The old “one inch per gallon” rule is a freshwater rule of thumb and does not work for marine fish, which need room to swim and territory to hold. Plan on adult size, temperament and where each fish lives in the tank, add two or three at a time at most, and leave at least two weeks between batches (Dr Tim’s guidance, Bulk Reef Supply, 2019).
What are the limits?
Other calculators
- Salt mixGrams of salt for a batch, and what your refractometer or hydrometer should read.
- Salinity correctionHow much water to swap, day by day, at a safe pace.
- Water change effectWhat a change, or a run of changes, does to a reading.
- ATO reservoir safetyHow far salinity falls if a stuck pump empties the reservoir.
- Dosing from a test resultMillilitres to bring alkalinity, calcium or magnesium back to target.
- Daily consumptionHow much your tank uses a day, and the daily dose to match.
- Unit converterdKH, meq/L, ppm, SG, ppt, litres and gallons, Celsius and Fahrenheit.
- Tank volumeLitres from the tank size, and the net water after rock, sand and sump.
- Heater sizeWatts for a UK room, split across heaters so one stuck on is not a disaster.
- Light hours and PARPhotoperiod, ramps, daily light and PAR by coral group.
- Electricity running costWhat the tank costs to run at the current UK price cap.