Betta Fish Tank Calculator (Size, Heater Watts, Filter Flow, Water Changes & Cycling)
Set up a betta tank properly: minimum and better tank sizes, heater wattage for a steady 25–27 °C in your room, a gentle filter flow that will not shred fins, water-change volume and frequency for the tank size and stocking, how long fishless cycling takes, tank-mate space rules, feeding pellets per day, and the running cost — with a sorority and divided-tank check.
How it works
minimum 5 gal for one betta (10 with tank mates, 20+ for a sorority) · heater ≈ 2.5 W/gal for a ≤ 5 °C rise, 4 W/gal to 10 °C, 5 W/gal beyond, to 25–27 °C · filter ≈ 3× tank volume per hour, baffled · water change ≈ 25% weekly, scaled by bio-load ÷ (gallons ÷ 5), × 0.6–0.8 with live plants · fishless cycle ≈ 4–6 weeks (3 heavily planted)
The betta in a vase is a marketing image, not husbandry: a tropical fish from warm, still, vegetated water needs a heated, filtered tank of a few gallons at least, with the filter's current tamed so that fins built for display rather than swimming are not battered against the glass. The heater is sized to the temperature lift the room demands, water changes to the load of fish on the volume with plants easing the burden, and a new tank must grow its nitrifying bacteria before a fish is asked to live in its waste. Tank mates are a matter of temperament and space, and a sorority of females is a project for the experienced rather than a default.
Worked example
A 5-gallon (19 L) tank for a single betta in a 20 °C room is the practical minimum: a 20 W adjustable heater lifts it the 6 °C to 26 °C, a sponge filter moving about 15 GPH keeps the current gentle, and with some live plants a 20% (≈ 4 L) change each week holds nitrate down; a new tank needs about 5 weeks of fishless cycling first. Four pellets a day in two meals, one fast day a week, and roughly $5 a month to run.
Frequently asked questions
Can a betta live in a bowl without a heater or filter?
It can survive for a while, which is why the myth persists, but unheated water below 24 °C suppresses its immune system and an unfiltered bowl swings in ammonia and temperature daily. Bettas in 5+ heated, filtered gallons live 3–5 years; bowl bettas rarely see two.
Which tank mates are safe?
Bottom-dwellers and small peaceful schoolers in 10 gal or more: corydoras, kuhli loaches, ember tetras, harlequin rasboras, nerite snails, and shrimp if the betta ignores them. Avoid anything colourful and long-finned (guppies), nippy (barbs, serpae tetras) or a rival labyrinth fish (gouramis).
Related calculators
Husbandry guidelines drawn from veterinary and keeper references (RSPCA, ARAV, species care sheets). Individual animals, breeds and climates vary — use these as starting points and confirm with an exotics veterinarian, especially for diet changes, temperatures and health concerns.