Betta Water Change Schedule: 25 Percent a Week, Never 100
Updated October 2026
For a filtered, cycled betta tank of five gallons or more, change 20 to 30 percent of the water once a week. That single rhythm keeps nitrates low and your betta stable, and it is the schedule most fishkeepers settle on after some trial and error.
The exact amount depends on tank size, filtration, and stocking, so here is how to dial it in.
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How Much Water to Change by Tank Size
The table turns this page’s schedule into gallons and litres. Read across from your tank size and filter.
| Tank size | Filtered | Percent and rhythm | Volume |
|---|---|---|---|
| 2 gal | Any | 30 to 50% twice a week | 0.6 to 1 gal (2.3 to 3.8 L) per change |
| 3 gal | Any | 30 to 50% twice a week | 0.9 to 1.5 gal (3.4 to 5.7 L) per change |
| 5 gal | Yes, cycled | 20 to 30% once a week | 1 to 1.5 gal (3.8 to 5.7 L) per change |
| 5 gal | No | 30 to 50% twice a week | 1.5 to 2.5 gal (5.7 to 9.5 L) per change |
| 10 gal | Yes, cycled | 20 to 30% once a week | 2 to 3 gal (7.6 to 11.4 L) per change |
| 10 gal | No | 30 to 50% twice a week | 3 to 5 gal (11.4 to 18.9 L) per change |
Percentages are the schedule from this page. Unfiltered rows reuse the small-tank rule because nothing is processing ammonia. Litres use 3.785 L per US gallon.
Betta Water Change Calculator
Assumes this page’s ranges: 20 to 30 percent weekly for a cycled, filtered tank of 5 gallons or more, 30 to 50 percent twice a week otherwise. Light feeding uses the low end, normal the middle, heavy the high end. Use the water volume you actually have, not the tank label.
The baseline schedule
A cycled 5-gallon tank with a gentle filter does well on one 25 percent change per week. Bump it to twice a week if your nitrates climb past 20 ppm between changes.
Smaller tanks swing faster and need more attention, which is one more reason a proper setup matters. Our betta tank size guide explains why five gallons is the sensible floor.
Why smaller tanks need more work
In a 2 or 3 gallon tank, waste concentrates quickly and parameters shift day to day. You may need 30 to 50 percent changes twice a week just to keep ammonia and nitrate in check.
That constant swing stresses the fish. If your betta hides or clamps its fins after changes, unstable water is a common cause, which we cover in why your betta is hiding.
How to change the water safely
Match the new water temperature to the tank within a degree or two, and always add dechlorinator before the water goes in. Sudden temperature or chemistry swings are harder on a betta than slightly higher nitrates.
Use a gravel siphon to pull waste from the substrate rather than just skimming the top. A gentle filter helps keep that waste in suspension, and our picks in best betta filters favor the low-flow models bettas tolerate.
Never change 100 percent
A full water change wipes out the beneficial bacteria living in your water and shocks the fish. Even a dirty tank should be corrected with a series of partial changes over a few days, not one complete swap.
The only exception is an emergency like a toxin spill, and even then you reintroduce conditioned, temperature-matched water carefully.
Reading your test kit
Let the numbers guide the schedule. Ammonia and nitrite should read zero in a cycled tank, and nitrate is your cue: when it drifts toward 20 to 40 ppm, it is time for a change.
Test weekly at first so you learn your tank’s rhythm, then you can ease off the testing once the pattern is clear.
