Saltwater Water Changes and Maintenance
🐠 The short version: water changes export waste and replenish minerals; topping off with fresh RO/DI holds salinity steady — they’re two different jobs, do both. A modest change every 1–2 weeks suits most tanks, mixed to the same salinity (~35 ppt) and temperature as the tank. Small and regular beats big and occasional, because stability is everything.
🔧 Mix the right amount of salt with the salt-mix calculator, and confirm the batch matches your tank on the salinity converter.
Why water changes matter
Even a well-run reef accumulates waste (nitrate, phosphate, organics) and slowly uses up minerals (alkalinity, calcium, magnesium, trace elements). A partial water change does two things at once: it exports some of the accumulated waste and replenishes the minerals — resetting the tank a little closer to fresh seawater. It’s the simplest, most reliable maintenance there is, and on a modestly-stocked tank it can do most of your chemistry work for you.
Top-off vs water change — not the same thing
These get confused constantly:
- Top-off replaces water lost to evaporation. Only pure water evaporates, leaving salt behind, so you top off with fresh RO/DI (no salt). Skip it and salinity creeps up. An auto top-off (ATO) automates this; otherwise make it a daily habit.
- Water change removes some tank water and replaces it with newly-mixed saltwater to export waste and replenish minerals.
You do both: top off constantly to hold salinity, and change water on a schedule to keep the tank clean and mineral-rich.
How to do a water change
- Mix the new saltwater ahead of time. Marine salt into RO/DI to about 35 ppt (SG 1.026) — let it mix and aerate (ideally for a while), then check it on the salinity converter. The salt-mix calculator tells you how much salt for your volume.
- Match temperature and salinity to the tank before it goes in — mismatches shock livestock.
- Siphon out the old water (a good moment to vacuum detritus from the sand and blow off the rocks first, so you remove the gunk you stir up).
- Add the new water gently.
How often, and how much
There’s no single correct number — it depends on stocking, feeding and your test results. A common, sensible routine is a modest change every 1–2 weeks. The principle that matters: small and regular beats large and occasional, because a big infrequent change swings parameters while small frequent ones keep them steady. Let your test numbers — especially nitrate and phosphate drifting up — tell you if you need more.
Do water changes replace dosing?
On a lightly-stocked or soft-coral tank, regular water changes usually replenish the alkalinity, calcium and magnesium your corals use — no dosing needed. As stony corals grow, their demand outpaces what water changes return, and you add dosing to hold alkalinity, calcium and magnesium steady between changes. Full picture in reef tank water chemistry explained.
A simple maintenance rhythm
- Daily: top off evaporation (or let the ATO do it); quick look at fish, temperature and equipment; feed sensibly.
- Weekly / fortnightly: test the key parameters, then a modest water change; clean the glass; empty/clean the skimmer cup if you run one.
- Monthly-ish: clean pumps and powerheads, replace filter socks/media as needed, check salinity calibration.
Keep it small, keep it regular, and the tank stays stable — which, as ever, is the whole game.
Sources & further reading
- Coralkin — Salt-mix calculator and Salinity converter — mix and confirm the right batch.
- Coralkin — Parameter chart — the targets your changes are maintaining.
- Coralkin — Reef tank water chemistry explained — when changes give way to dosing.
- Bulk Reef Supply — water changes, salt mixing and maintenance.
Common questions
How often should I do a water change?
A common, sensible routine is a modest change every one to two weeks, but there is no single magic number - it depends on stocking, feeding and your test results. Small and regular beats large and occasional, because it keeps parameters steady rather than swinging them.
Whats the difference between a water change and topping off?
Topping off replaces water lost to evaporation, and you use fresh RO/DI with no salt (salinity rises as water evaporates, so you add pure water back). A water change removes some tank water and replaces it with newly-mixed saltwater to export waste and replenish minerals. They are different jobs - do both.
How do I mix saltwater for a change?
Mix marine salt into RO/DI water to about 35 ppt (SG 1.026), let it mix and aerate, and bring it to the same salinity and temperature as your tank before adding it. Matching salinity and temperature avoids shocking your livestock.
Do water changes replace dosing?
On a lightly-stocked or soft-coral tank, regular water changes usually replenish the alkalinity, calcium and magnesium corals use. As stony corals grow, their demand outpaces water changes and you add dosing to hold the numbers steady between changes.
Why is my salinity creeping up?
Almost always missed top-offs. As water evaporates the salt stays behind, so salinity climbs unless you replace evaporation with fresh RO/DI. An auto top-off, or a consistent daily top-off habit, keeps it stable.