Saltwater water changes keep your reef stable and clean. They replace trace elements and dilute waste. A calm, repeatable routine prevents most problems.
Plan the water change and mix new saltwater
Pick a change size you can repeat every week. Many hobbyists do 10% weekly. Others do 15% every two weeks. Consistency matters more than big swings.
Mix new saltwater at least 12 to 24 hours ahead. Use RO/DI water when possible. Aim for salinity of 1.025 to 1.026 at 77°F. Match the tank temperature within 1°F.
Add salt to water, not water to salt. Use a powerhead for strong circulation. Heat the mix with a spare heater. Test salinity with a calibrated refractometer.
Check key parameters before you start. Target alkalinity 7.5 to 9.0 dKH for most reefs. Keep calcium near 400 to 450 ppm. Keep magnesium near 1250 to 1400 ppm. If your new mix is far off, adjust slowly over several changes.
- Label a dedicated mixing bin and hose for saltwater only.
- Calibrate your refractometer with 35 ppt solution monthly.
- Mix to 35 ppt, then fine-tune after the water reaches tank temperature.
If you need help choosing a schedule, see our reef tank maintenance schedule. If your salinity drifts often, review how to measure salinity.
Perform the change safely without stressing livestock
Turn off equipment that can run dry. Shut down the return pump and skimmer first. Keep powerheads on if the water level stays safe. This maintains oxygen during the work.
Siphon water into a marked container to avoid over-draining. For a 40-gallon tank, a 10% change is 4 gallons. Use the siphon to remove detritus from bare spots. Avoid deep sand digging, which can release trapped waste.
Clean as you siphon. Wipe the glass line and blow debris off rocks with a turkey baster. Pull out a filter sock and swap it now. Replace mechanical media after the change, not before.
Add new water slowly to prevent swings. Pour into the sump if you have one. Use a small pump and tubing for control. Restart the return pump, then the heater, then the skimmer. Expect the skimmer to surge for an hour.
- Match salinity within 0.001 specific gravity of the display.
- Keep the change water within 1°F of the tank.
- Add water over 5 to 15 minutes for tanks under 75 gallons.
After the change, test salinity again. Check alkalinity if you keep stony corals. Log results in a notebook or app. Trends matter more than single readings.
Troubleshooting and common mistakes to avoid
Cloudy water after a change often means precipitation. This can happen if you mix too hot or add salt too fast. It can also occur with old salt mix exposed to moisture. Mix at 76 to 78°F and keep salt sealed.
Fish breathing fast can signal low oxygen or a temperature swing. Point a powerhead at the surface for agitation. Verify the heater did not stay off. Confirm the new water was fully aerated during mixing.
Alkalinity swings are a frequent reef mistake. Some salt mixes run 10 to 12 dKH. If your tank runs 8 dKH, big changes can shock SPS. Use smaller changes like 5% twice weekly. Or pick a salt closer to your target.
Never rely on “bucket math” alone. Measure the drained volume the first few times. Mark a line on your sump for the normal water level. For nitrate control, pair changes with good export. Our nitrate control guide can help.
- Do not change more than 20% at once in a mixed reef.
- Do not vacuum deep sand beds deeper than 1 inch.
- Do not add unmixed salt directly to the aquarium.
Sources: Instant Ocean Sea Salt mixing guidelines; Red Sea Salt mixing guidance; Randy Holmes-Farley, Reefkeeping Magazine articles on water changes and seawater chemistry.
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