Help and extras
Calculator results are planning help, not a guarantee. Individual fish and tanks vary. Read every check, then double-check important decisions. You stay responsible for your animals, equipment, and aquarium. FitMyTank is not responsible for losses, injury, livestock death, equipment damage, water damage, or other consequences from using these results. Read full disclaimer.
How to use this calculator
- Enter your tank size in litres or gallons.
- Search for freshwater fish by common or scientific name.
- Add each species you want to compare.
- For Check My Plan, set how many of each with + and -.
- Click Check My Plan for that exact mix, or Optimize My Tank if you want us to suggest numbers.
- Read how full the tank is, and every other check, before you buy.
Need a quick overview first? Visit the Freshwater Aquarium Tools homepage.
What this calculator checks
- TemperatureThe fish should like a similar temperature.
- pHThe fish should like a similar pH.
- Water hardnessSome fish like softer water, others harder. That is checked on its own.
- Group sizeSchooling fish often do better in a proper group.
- Do they get along?Fish can like the same water and still not mix well.
- Tank sizeYour tank is compared with the stored minimum for each fish. A big enough tank is not the same as how full it is.
- How full is the tank?The calculator estimates how packed the tank would be. The numbers stay under How FitMyTank calculated this.
Why all seven checks matter
Two fish can like the same temperature or pH and still not work in the same tank. Group size, behavior, water hardness, and tank size can change the answer.
How to read your results
No known problem found by that check.
Read that check before you buy.
This mix fails that check.
We can't give a reliable answer yet.
A green result on one check does not mean the whole mix is fine. Read all seven.
What this calculator cannot do
This is planning help based on stored data. It cannot see your actual fish, tank layout, health, or unusual setup. Use the results, then still look each species up and watch the tank in real life.
Helpful equipment (coming soon)
Future updates will include practical equipment suggestions linked to your results.
- Water testing basics
- Heating and temperature monitoring
- Filtration and flow support
Common questions
How does the calculator work?▼
It checks seven things: temperature, pH, water hardness, group size, whether the fish get along, tank size, and how full the tank is.
Can this tell me exactly how many fish I can keep?▼
No. It is a planning tool. A good group size is not the same as how full the tank is. Tank size is also a separate check.
Does compatibility depend only on tank size?▼
No. Fish can still be a bad match because of water, group size, or behavior even when the tank is big enough.
Why can one check look good while another fails?▼
Each check looks at a different problem. Read all seven before you buy.
What happens when there is not enough data?▼
That section says we don't have enough reliable data yet, so you can look that part up before stocking.
Does the calculator check schooling?▼
Yes. Group size is compared with stored minimums where we have solid data.
Does the calculator check fish behavior?▼
Yes. Whether they get along is checked separately from water.
Can freshwater fish with different pH needs live together?▼
Sometimes, if their pH ranges overlap. If they don't, pick one of the two species for this tank.
Is this a replacement for researching each species?▼
No. Use it for a quick practical answer, then still check care details before you buy.