What size heater and filter does your aquarium need? Enter the tank volume, your coldest room temperature and the temperature your fish need, and the calculator suggests heater wattage, filter flow and, for planted tanks, lighting and CO₂ notes.
How it sizes the equipment
- Heater: a common rule is 3–5 watts per gallon, more when the room is cold. The calculator scales it with the gap between room and tank: about 2.5 W per gallon plus 0.2 W for every °F of difference, a little less per gallon above 60 gallons and 20% more with an open top. Above about 75 gallons it suggests two heaters, so one failing does not cool or cook the tank.
- Filter: aim to turn the water over 4–6 times an hour for community tanks, 6–10 for planted tanks and 8–10 for goldfish. Bettas and shrimp prefer gentle flow, so it caps at about 80 gallons per hour.
- Box flow vs. real flow: filter ratings are measured empty, without media or lift, so the calculator suggests buying about 40% above your target.
A worked example
A 20 gallon community tank in a room that drops to 68 °F, kept at 77 °F, needs about 86 W of heating, so a 100 W heater. The filter should move 80–120 gallons an hour, which means a filter rated around 115–170 GPH on the box.
Read more
- Beginner fish that actually survive a first tank.
- Goldfish tanks built for the bioload (goldfish need far more filtration).
- Peaceful community tank combos.
These calculators use standard formulas and common aquarium guidelines. Fish, plants and rooms vary, so treat the results as a starting point and watch your tank.
Frequently asked questions
What size heater do I need for a 20 gallon tank?
Usually 75–100 W in a normal home, more in a cold room or with an open top. Size for the coldest night, not an average day.
How much filtration does a fish tank need?
Most community tanks do well with 4–6 times the tank volume per hour. Planted tanks often run 6–10 times, goldfish tanks 8–10, and betta or shrimp tanks much gentler.
Is it better to use two heaters?
For large tanks, yes. Two smaller heaters spread the heat and mean one stuck on or off is less likely to harm your fish.
Do bettas need a filter?
Yes, but a gentle one. A sponge filter or an adjustable filter with a baffle keeps the water clean without blowing a betta around the tank.
Is my information stored?
No. Everything is calculated in your browser, and nothing you type is saved or sent anywhere. The share link keeps your numbers in the part of the web address after the # sign, which browsers do not send to the website.