Reef tank water changes keep nutrients in check. They also replace trace elements that corals use each day. A good routine prevents many common problems.
Why water changes matter in reef tanks
Water changes export nitrate, phosphate, and dissolved waste. Skimmers and filters help, but they miss some compounds. A steady change schedule keeps chemistry predictable.
New saltwater restores alkalinity, calcium, and magnesium. It also adds minor elements like iodine and strontium. These get consumed by corals and coralline algae.
Most mixed reefs do well with 10% weekly changes. Another option is 20% every two weeks. Keep changes smaller if you chase stability.
Use numbers to guide the plan. Aim for salinity 1.025–1.026 at 77°F. Keep alkalinity 7.5–9.0 dKH, calcium 400–450 ppm, and magnesium 1250–1400 ppm.
- Soft coral tanks: 10% weekly is usually enough.
- SPS-heavy tanks: 5–10% weekly supports stability.
- High nutrients: add one extra 10% change for two weeks.
If you battle algae, do not rely on water changes alone. Fix feeding, export, and flow too. See reef tank nitrate control for a full plan.
Prep the new saltwater the right way
Mix water at least 4–24 hours before the change. Use RO/DI water with 0 TDS. This prevents silicates and metals from sneaking in.
Use a clean mixing bin and a heater. Add a powerhead for strong circulation. Bring the water to 76–78°F before you match salinity.
Add salt to water, not water to salt. Start with about 1/2 cup of salt per gallon. Then adjust with a refractometer or calibrated digital meter.
Match salinity within 0.001 specific gravity of the display. Match temperature within 1°F. This reduces stress on fish and coral tissue.
- Calibrate refractometers with 35 ppt solution, not RO/DI.
- Label your mixing pump and hose for saltwater only.
- Test alkalinity of new mix to avoid surprise swings.
If your alkalinity runs 8.5 dKH, keep new water close. A jump from 7.0 to 10.0 can shock SPS. If needed, blend two salt mixes for consistency.
Step-by-step: doing the water change
Start by turning off heaters in the sump. Turn off the return pump and skimmer. Leave powerheads on in the display for oxygen.
Mark the sump waterline before you drain. This helps you refill to the right level. For a 75-gallon system, a 10% change is about 7–8 gallons.
Use a siphon hose to remove water from the sump first. Then vacuum small sections of sand if needed. Do not deep-stir the bed in one session.
Drain into a bucket with a measured volume. Stop at the planned amount. Then pump new saltwater into the sump slowly.
- Change 5–10% at a time for sensitive coral tanks.
- Refill over 5–10 minutes to avoid salinity spikes.
- Restart return pump, then skimmer after 10 minutes.
Watch the skimmer after restart. It may overflow for an hour. Keep the cup drain open or lower the water level temporarily.
After the change, test salinity and temperature first. Then check alkalinity if you keep SPS. Log results in a notebook or app for trends.
For more stability tips, review reef tank salinity stability. If you need gear ideas, see reef tank maintenance checklist.
Troubleshooting and common mistakes
Cloudy water often comes from unmixed salt or disturbed sand. Mix longer and refill slower next time. Use filter floss for a few hours if needed.
Corals closing can mean temperature or salinity mismatch. Check both right away. Also check alkalinity if you changed to a new salt brand.
Rising nitrate after changes can happen with dirty socks or detritus traps. Clean filter socks within 2–3 days. Vacuum one small sump area each change.
Never add salt directly to the display to “top off.” That can burn tissue and spike salinity. Top off only with fresh RO/DI water.
Sources: Instant Ocean Sea Salt Guide (manufacturer instructions); Red Sea Reef Care Program (water parameter targets); Randy Holmes-Farley, Reefkeeping Magazine (reef chemistry articles).
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