Overview
The water change is the oldest and most reliable tool in reefkeeping. It exports nutrients, replenishes trace elements and alkalinity, and dilutes the things your test kit does not measure. Here is how much to change, how often, and why it still matters even on a heavily-automated tank.
How Much and How Often
A typical schedule is a 10–20% change every one to two weeks. Smaller, more frequent changes keep parameters more stable than occasional large ones, which is why many reefers prefer weekly. The right amount depends on your stocking, feeding, and what other export methods you run — a lightly-stocked tank with a strong refugium needs less than a heavily-fed fish system.
How to Do a Water Change Properly
- Mix quality salt in RODI water and let it circulate and heat to tank temperature ahead of time.
- Match the new water’s salinity to the tank before adding it.
- Siphon detritus from the sand and rockwork as you remove old water — this is half the value of the change.
- Add the new water gently so you do not shock livestock with a sudden swing.
Do You Still Need Them If You Dose?
Yes. Dosing replaces the big-three (alkalinity, calcium, magnesium), but water changes also restore the dozens of minor and trace elements corals consume, and they dilute organics and pollutants no test measures. Automated tanks can stretch the interval, but water changes remain the simplest insurance against the unknown.
Common Mistakes
Adding cold or freshly-mixed (un-aerated) salt water, changing so much at once that parameters swing, and forgetting to match salinity are the usual culprits. Consistency and matching your new water to the tank are what make water changes safe.
Related Guides
- Alkalinity, Calcium & Magnesium
- Salinity & Specific Gravity
- Why RODI Water Matters
- Lowering Nitrates & Phosphates
Frequently Asked Questions
How much water should I change and how often?
A common approach is 10 to 20% every one to two weeks. Smaller, more frequent changes hold parameters steadier than large occasional ones, so adjust the amount to your stocking and nutrient levels.
Do I still need water changes if I dose the big-three?
Yes. Dosing handles alkalinity, calcium, and magnesium, but water changes replenish trace elements and dilute organics and pollutants that dosing and test kits miss. They remain valuable insurance even on an automated tank.
What salt and water should I use for changes?
Use a quality reef salt mixed in RODI (not tap) water, pre-mixed and heated to tank temperature and matched for salinity. Clean source water and matched parameters are what make a water change safe.