Water Changes Checklist for Reef Keeping

For stable mixed reefs, 5 to 10 percent weekly or 10 to 15 percent every two weeks is common. If nitrate is above 20 ppm or phosphate is climbing past 0.10 ppm, smaller repeated changes are usually safer than one massive correction.

Measure display tank salinity with a calibrated refractometer or reliable conductivity meter and target the new water to match within 0.001 SG. Matching 1.025 to 1.026 SG closely helps prevent osmotic stress in fish, shrimp, and sensitive SPS colonies.

Test dKH, calcium, and magnesium so the new saltwater does not create a major swing. Aim to keep alkalinity changes within about 0.3 to 0.5 dKH in a single water change, especially in Acropora-dominant systems.

High-alkalinity salt mixes can be useful for fast-growing coral systems, but they may shock low-nutrient tanks if your display runs significantly lower dKH. Consistency matters more than chasing a brand change every bucket.

Use 0 TDS RODI water whenever possible to avoid importing silicates, nitrate, phosphate, copper, or chloramine-related contaminants. Running short halfway through a water change often leads to rushed shortcuts that cause instability.

Avoid making large changes to two-part, kalkwasser, or calcium reactor output on the same day. Changing multiple variables at once makes it harder to identify the cause if corals retract or alkalinity shifts unexpectedly.

Use a dedicated mixing pump and heater to bring the new water close to display temperature, usually 76 to 79 F. Fully dissolved saltwater with stable temperature reduces the chance of precipitation and temperature shock.

Pouring water onto dry salt can cause localized precipitation and inconsistent chemistry. Add salt gradually to the full volume of RODI water while a pump is running for a more even mix.

Most reef keepers benefit from mixing for at least 4 to 24 hours depending on the salt brand. This gives pH, oxygenation, and salinity time to stabilize before the water enters the tank.

A narrow temperature match helps prevent fish stress and reduces the chance of coral slime response. This is especially important for LPS and Euphyllia that react quickly to sudden thermal shifts.

Salt mixes can read differently before they finish dissolving, so always confirm final SG after mixing is complete. Target within 0.001 SG of the display to avoid a noticeable swing during a larger change.

Freshly mixed water can vary significantly by brand and even by bucket if the dry mix settled during shipping. Knowing the batch dKH helps you estimate how the water change will affect an SPS-heavy system.

Shutting down key equipment prevents the sump from running dry, stops the auto top off from adding freshwater incorrectly, and avoids skimmer overflows when water level changes. Label switches or use a controller feed mode if available.

Residue from soap, cleaners, or metal contamination can wipe out invertebrates quickly. Use food-safe or aquarium-only equipment and rinse with RODI water between uses.

Target sump chambers, bare-bottom corners, behind rockwork, and dead spots where mulm accumulates. Exporting detritus during a water change is often more useful than simply replacing water alone.

If the tank has an older deep sand bed, siphon only the surface in small sections rather than stirring deeply. Over-disturbing anaerobic zones can release trapped nutrients and sulfide pockets.

Look for signs of stress such as gaping LPS mouths, fish heavy breathing, RTN on SPS tips, or recent dosing errors. If livestock already appears unstable, make a smaller change and avoid abrupt corrections.

Exposed heaters can crack and pH or salinity probes can give false readings if left dry. Plan the drain level so equipment stays safe or remove sensitive devices before lowering the water line.

Mark buckets or use graduated containers so the replacement amount is accurate. Estimating by eye can create salinity drift over time, especially in tanks that already run auto top off and heavy evaporation.

Adding new saltwater too quickly can temporarily blast corals with different alkalinity, temperature, or pH. A small utility pump or slow siphon into the sump is usually the gentlest approach.

Do not pour directly onto Euphyllia, acans, scolymia, or open brains because the flow can damage tissue. Aim the refill into a filter sock chamber, sump, or high-dispersion area instead.

Bring the return pump online first, then wavemakers, then skimmer after the water level stabilizes. This sequence helps purge air, confirms normal sump level, and reduces messy skimmer overflow.

If the ATO turns back on too early, it can dilute the system based on temporary sump level changes. Wait until flow is normalized and then verify SG before restoring automation.

A little polyp retraction can be normal, but severe mucus, rapid breathing, or fish flashing may signal a mismatch in temperature, salinity, or contaminants. Early observation gives you a chance to intervene before stress escalates.

Final salinity can shift once the sump and display equalize, so confirm the system settled where expected. Keeping reef salinity around 1.025 to 1.026 SG consistently is more important than chasing tiny daily changes.

High-consumption systems can show meaningful dKH movement after even a moderate water change. Confirm the tank remains within your normal operating range, often 7.0 to 9.0 dKH depending on nutrient strategy.

One water change rarely solves chronic nutrient problems if feeding, export, or detritus control remain unchanged. Track whether nitrate and phosphate actually decline over two to four weeks before increasing water change volume.

A water change often dislodges waste into the water column, and dirty socks can quickly trap and decompose it. Replace or wash them within 24 hours to prevent nutrients from bouncing back.

If alkalinity or calcium rises after each change, your salt mix may be supplying more than the tank consumes. Make small dosing corrections based on a pattern, not a single test result taken right after maintenance.

Recording SG, dKH, nitrate, phosphate, and any livestock response helps identify trends such as a bad salt batch or an overly aggressive change schedule. This information becomes invaluable when troubleshooting coral color loss or unexplained algae growth.