Water changes dilute nutrients and restore depleted ions. They also introduce whatever is in your salt mix, so matching parameters matters.
How much and how often?
Many reefs do 10–20% every 1–2 weeks; heavily dosed tanks may change less. Holmes-Farley modelled how water changes slowly counter magnesium depletion, but daily dosing is still needed in high-demand tanks (Randy Holmes-Farley, "Water Changes in Reef Aquaria," Reefkeeping Magazine).
Match the new water
- Salinity within 0.5 ppt
- Temperature within 1 °F / 0.5 °C
- Alkalinity ideally within 0.5 dKH of the tank
Salt mix differences can cause alk swings (alk swings; salt guide).
Steps
- Mix salt in RO/DI and circulate/heat for 12–24 hours.
- Test salinity and alk.
- Siphon detritus while removing water.
- Add new water slowly.
Sources
FAQ
Can I skip water changes? Some tanks do with full dosing, but you lose dilution of unknown buildups.
Should I use tap water? No; use RO/DI (RO/DI guide).