A common routine is about 10% of your water every week, or 20% every two weeks. Both change the same amount of water each month, and they keep the tank at almost the same level. Choose the one you’ll actually keep up, then adjust it using your nitrate, phosphate and alkalinity trends.
What a water change actually does
A water change does two jobs. It dilutes things that build up, such as nitrate, phosphate and the waste a skimmer misses. And it tops up things that get used, such as alkalinity, calcium, magnesium and trace elements, back towards the level your salt mixes to.
A change removes the same percentage of a dissolved substance as the percentage of water you take out. Bulk Reef Supply makes the same point about nitrate: a 10% change takes out about 10%.
The maths: little and often vs bigger and less often
Say your tank adds 1 mg/L of nitrate a week and nothing else removes it. With only water changes to dilute it, nitrate settles where what you remove each time equals what’s been added since the last change.
| Routine | Water per month | Before each change | After each change |
|---|---|---|---|
| 5% weekly | About 20% | 20 mg/L | 19 mg/L |
| 10% weekly | About 40% | 10 mg/L | 9 mg/L |
| 20% every 2 weeks | About 40% | 10 mg/L | 8 mg/L |
| 20% weekly | About 85% | 5 mg/L | 4 mg/L |
Simple dilution: the level just before a change = what builds up between changes ÷ the fraction changed.
What the numbers tell you
Weekly 10% and fortnightly 20% settle at almost the same level. The amount of water you change each month matters much more than how you split it up. And halving the change size doubles the level things settle at.
Real tanks also remove waste through the skimmer, bacteria and algae, so actual levels are lower than this. The pattern still holds.
Choosing your routine
Start with an amount you can keep up with week in, week out. A routine you skip every third time does less than a smaller one you never miss.
- New tanks: many keepers change more often in the first months, while the tank is settling.
- Heavily stocked or fed tanks: more waste means more to dilute. Bigger or more frequent changes help, alongside good filtration.
- Tanks that dose: water changes put back only what your salt contains. If you dose alkalinity, a big change can briefly move it towards your salt’s level.
- Light-bioload soft coral tanks: often fine with smaller or less frequent changes.
Use your tests to adjust
Your test results tell you whether the routine is right. If nitrate and phosphate climb over a month, you need more export: more water changed, or better filtration. If they’re steady and your corals look good, the routine is working.
Change one thing at a time and give it a month before judging. Log your tests so you can compare the month before and after.
Make it a habit
Pick a day, mix the water the day before if your salt allows it, and keep the kit together. Set a repeating task in the Companion and it shows the change as due on the right day. If you have an account, it can email you that morning too. When you do it late, the next one is counted from the day you actually did it.
For the step-by-step method, see how to do a water change. To plan how much salt you’ll use over a year, try the saltwater planner.
Common questions
Is a 10% weekly water change enough?
For many reef tanks, yes. Check it against your nitrate and phosphate trend over a month.
Can I do a 50% water change?
It’s possible with well-matched water, but it moves everything at once. Smaller, regular changes are gentler. Large changes are mainly for emergencies.
Can I skip water changes if I dose?
Some keepers do, using other methods to remove waste and replace trace elements. It needs more testing and experience.
Sources and limits
Last checked 12 October 2026. The examples use simple dilution maths and made-up numbers. Real tanks also remove waste through skimming, bacteria and algae.
Keep going
Set your water change schedule · Free Reef Companion · All calculators