In a cycled aquarium, bacteria turn fish waste into nitrate. Unlike ammonia and nitrite, nitrate isn't removed by the filter: it builds up until you take it out, and so do dissolved organics, while the minerals that buffer pH are slowly used up. Plants absorb some nitrate, but in most tanks water changes are the only real way to reset the water. The question is how much, and how often.
The maths of a water change
When you replace a fraction p of the water, you remove that fraction of the nitrate and add tap water with its own nitrate:
nitrate after = nitrate before × (1 − p) + tap nitrate × p
With nitrate-free tap water, a 25% change takes 40 ppm down to 30 ppm; a 50% change takes it to 20 ppm. Because each change removes a share of what is left, small changes add up more slowly than you might expect: two 25% changes remove 44% of the nitrate, not 50%, and it takes five of them to remove as much as two 50% changes.
Where a weekly routine settles
Suppose your tank's nitrate rises by 10 ppm a week. If you change a fraction p every week, nitrate settles into a steady pattern: it peaks just before each change at tap + 10 ÷ p and drops just after to tap + 10 × (1 − p) ÷ p.
| Weekly change | Low (after change) | Peak (before change) |
|---|---|---|
| 10% | 90.0 ppm | 100.0 ppm |
| 20% | 40.0 ppm | 50.0 ppm |
| 25% | 30.0 ppm | 40.0 ppm |
| 30% | 23.3 ppm | 33.3 ppm |
| 40% | 15.0 ppm | 25.0 ppm |
| 50% | 10.0 ppm | 20.0 ppm |
That is the clearest argument for regular, generous changes: with a 10 ppm weekly rise, 10% changes let nitrate settle around 90–100 ppm, while 30% changes hold it near 23–33 ppm. Measure your own weekly rise by testing just after a change and again a week later.
How often for different tanks
- Lightly stocked community or planted tank: 20–30% weekly, or 30–50% every two weeks if nitrate stays low.
- Typical community tank: 25–35% weekly.
- Goldfish, cichlids and heavily stocked tanks: 40–50% weekly, sometimes twice a week.
- Nano tanks under 10 gallons: 25–30% weekly; small volumes change fast.
- Shrimp tanks: smaller, more frequent changes (10–20%) with carefully matched water, because shrimp are sensitive to sudden shifts.
Most freshwater fish do well with nitrate under 40 ppm; sensitive species, fry and invertebrates are happier under 20 ppm. The water change schedule turns these into percentages for each kind of tank.
Step by step
- Unplug heaters and filters that could run dry.
- Siphon out the planned amount with a gravel vacuum, cleaning one section of substrate each time.
- Prepare new water at the tank's temperature (within 2 °F / 1 °C) and add dechlorinator dosed for the new water, or for the whole tank if your conditioner allows it.
- Refill slowly onto a plate, rock or your hand to avoid stirring up the substrate.
- Plug everything back in and check the temperature after an hour.
Use the dosing calculator to scale your conditioner to the new water volume.
Large changes and old tank syndrome
If a tank has gone months without changes, nitrate may be over 100 ppm and pH may have drifted down as the buffering minerals were used up. Fish adapted to that water can be shocked by a sudden 50–80% change. Bring it back over a week or two: 20–25% every day or two, testing as you go. Once the tank is healthy, large changes are safe as long as temperature is matched and chlorine is neutralized.
Know your tap water
Test your tap water for nitrate at least once. Some municipal supplies and many wells carry 10–40 ppm, which sets a floor no water change can go below — the calculator shows this as an unreachable target. In that case, live plants, reduced feeding, or blending with RO (reverse osmosis) water are the way to lower nitrate further. Also check whether your supply uses chloramine, which needs a conditioner that handles it.
Common mistakes
- Topping off instead of changing: adding water to replace evaporation concentrates minerals and does nothing for nitrate.
- Rinsing filter media under the tap: chlorine kills beneficial bacteria. Rinse sponges in the water you removed.
- Changing all the water: 100% changes and deep cleaning everything at once remove bacteria and shock fish. Except in emergencies, stay under about 50% per change.
Plan your routine with the water change calculator: enter your nitrate readings to see how many changes it takes to reach your target.