The short answer
For most freshwater tanks, change 25–30% of the water once a week. It’s the single most valuable habit in the hobby — more effective than any additive or gadget at keeping fish healthy and algae at bay. A weekly change exports nitrate and other dissolved waste before it builds up, and tops up trace minerals plants and fish use.
Why weekly, and why that amount
Your filter converts fish waste into nitrate, but it can’t remove it — only a water change does that. Let nitrate climb and you get tired fish, stunted growth and algae blooms. Around 25–30% weekly keeps nitrate in a safe range for a typical community tank without causing the big swings a huge change can.
When to change more (or less)
- More often / more volume: heavily-stocked tanks, messy fish (goldfish, cichlids), during a fish-in cycle, or if a test shows high nitrate.
- Less often: lightly-stocked, well-planted or larger tanks stay stable longer — but test first before stretching the interval.
The honest way to know is to test your nitrate. If it’s creeping above ~20–40 ppm between changes, do more; if it stays low, your routine is working. A liquid test kit takes the guesswork out.
Make it easy
The easier a water change is, the more reliably you’ll do it. A gravel vacuum or hose-fed water changer turns it into a ten-minute job and cleans the substrate at the same time. For the full method, see how to do a water change, and browse all maintenance gear.