Most reef keepers change 10–20% of their water at a time. The amount you change each month matters more than the size of each change, so pick a size you can match carefully and keep up with. Large changes of 30% or more move salinity, temperature and alkalinity at once, so save them for emergencies.
Litres for your tank
Use your real water volume, sump included, not the size on the box. The tank volume calculator works it out.
| Tank volume | 10% | 15% | 20% | 25% |
|---|---|---|---|---|
| 60 L | 6 L | 9 L | 12 L | 15 L |
| 100 L | 10 L | 15 L | 20 L | 25 L |
| 200 L | 20 L | 30 L | 40 L | 50 L |
| 300 L | 30 L | 45 L | 60 L | 75 L |
| 500 L | 50 L | 75 L | 100 L | 125 L |
What each change removes
A change removes the same percentage of a dissolved substance as the percentage of water you change. Bulk Reef Supply makes this point about nitrate: a 10% change takes out about 10%.
So two 10% changes don’t equal one 20% change: they remove 19%, because the second change works on water that’s already been diluted. In practice the difference is small; what matters is the total over a month. See how often to change water for the maths.
Why bigger isn’t always better
The bigger the change, the more any difference between your new water and the tank shows. If your salt mixes to 7 dKH and your tank runs at 9 dKH, a 30% change drops alkalinity by about 0.6 dKH in an hour. The same goes for salinity and temperature.
Match the new water carefully: Red Sea says to check the salinity of both the new batch and the tank before adding it. The bigger the change, the more this matters.
When a big change makes sense
A large change is the fastest way to dilute something harmful: an overdose, a crashed parameter or a chemical spill. In those cases, match temperature and salinity carefully and do it, then go back to your normal routine.
Common questions
Is 10% a week enough?
For many reef tanks, yes. Check your nitrate and phosphate trend over a month.
Can I do a 50% water change?
Only with well-matched water, and usually as an emergency step.
Should I change more in a new tank?
Many keepers change more often in the first months, but use RODI water so you don’t feed diatoms.
Sources and limits
Last checked 12 October 2026. The maths is simple dilution. Real tanks also remove waste through skimming, bacteria and algae.
- Bulk Reef Supply: 5 ways to lower nitrates
- Red Sea: how to mix salt
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
Keep going
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