How Often to Change Betta Tank Water
Short answer: in a cycled 5-gallon tank, change 25–50% of the water once a week. Smaller or uncycled tanks need more frequent changes. Always use dechlorinated water at the same temperature as the tank.
Why water changes matter
Fish waste breaks down into ammonia, which a cycled filter converts to less harmful nitrates. But nitrates still build up over time. Water changes are how you remove them and replace trace minerals. Skipping changes lets nitrates climb, which stresses the fish and invites disease.
Think of it as cleaning the air the fish breathes — filters handle some, but regular changes are non-negotiable.
How much and how often
| Tank setup | Change amount | Frequency |
|---|---|---|
| Cycled 5-gallon, single betta | 25–50% | Once a week |
| Smaller tank (under 5 gal) or uncycled | 50% | 2–3× a week |
| 10+ gallon, planted | 25% | Every 1–2 weeks |
If your water test shows high nitrates, ammonia, or nitrite, change more often until it reads safe levels.
The step-by-step water change
- Prepare new water in a clean bucket. Treat it with a dechlorinator (conditioner) to remove chlorine and chloramine.
- Match the temperature. Aim for the same 78–80°F (25–27°C) as the tank — a sudden change shocks the fish.
- Turn off the filter and heater while you work.
- Siphon out 25–50% from the bottom to lift up uneaten food and waste. Use a gravel siphon or a turkey baster.
- Slowly add the new water back into the tank.
- Restart the equipment and check the thermometer.
Golden rules: never change 100% of the water at once (it shocks the fish and kills the beneficial bacteria), and never add untreated tap water. Dechlorinator is not optional.
Signs the water needs changing
- Cloudy or foul-smelling water.
- Uneaten food sitting on the bottom.
- A fish that is lethargic, gasping, or breathing fast.
- Test results: ammonia or nitrite above 0, or nitrate above 20–40 ppm.
Related guides: What to feed your betta · How to cycle the tank · Complete beginner guide