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Reef Tank Water Changes: How Much, How Often
A typical mixed reef benefits from a 10–15% water change weekly, using new saltwater mixed to the same salinity and temperature as the display. Water changes dilute accumulated nitrate and phosphate, replenish trace elements, and give alkalinity, calcium and magnesium a small buffer top-up — but only if the new water matches the tank closely enough that it doesn't create its own swing. A consistent schedule matters more than the exact percentage you pick.
A water change is the oldest maintenance task in the hobby, and also one of the easiest to get slightly wrong. Done well, it is a gentle, steady correction: nutrients get diluted, trace elements get topped up, and the buffer system gets a small nudge in the right direction. Done carelessly — mismatched salinity, cold water straight from the mixing bin, a huge change after weeks of neglect — it can push a stable tank into stress instead of relieving it. The mechanics are simple, but the details are what separate a routine that helps from one that quietly works against you.
What a water change actually does
A partial water change replaces a fraction of your tank's water with freshly mixed saltwater. The most obvious benefit is nutrient export: removing a portion of the water removes a proportional share of accumulated nitrate, phosphate and dissolved organics along with it. Less obvious, but just as important, is replenishment — new saltwater carries a full set of trace elements at their original concentration, some of which get consumed by corals faster than dosing alone tends to replace. A water change also gives alkalinity, calcium and magnesium a small top-up, though it is not a substitute for dedicated dosing on a heavily stocked tank.
What a water change does not do is fix an acute problem instantly. Diluting nitrate from 20 ppm to 17 ppm with a 15% change is real progress, but it will not resolve a nutrient imbalance that took weeks to build up. Think of it as steady maintenance rather than an emergency lever, and reach for testing and targeted dosing when something needs a faster correction.
How much and how often
There is no single correct percentage — the right schedule depends on bioload, feeding, and how much dosing or nutrient export equipment (skimmer, refugium, GFO) the system already carries. The table below compares the three schedules most reef keepers settle on.
| Schedule | Best for | Trade-off |
|---|---|---|
| 10–15% weekly | Most mixed reefs; the default recommendation | A comfortable balance of nutrient export and effort; easy to stay consistent with. |
| 5% twice weekly | Heavily stocked or SPS-dominant tanks wanting smoother dilution | Smaller swings per change, but doubles the mixing and testing workload. |
| 20–25% every two weeks | Lower-maintenance systems with strong skimming and dosing already in place | Fewer sessions, but each change has a bigger effect if salinity or temperature is off. |
Whichever cadence you choose, consistency is the variable that actually matters. A tank on a reliable 12% weekly schedule will generally out-perform one that swings between a rushed 5% one week and a guilty 30% the next, even though the second tank might average a similar total volume changed.
Matching salinity and temperature
The single most common way a water change goes wrong is adding new saltwater that does not match the display tank closely enough. A litre of water at the wrong salinity or several degrees off temperature is a small, sudden shock to every animal in the tank, even at a modest change percentage. Mix new saltwater to the same salinity as your display — roughly 35 ppt / 1.025–1.026 SG for most mixed reefs — and check it with a properly calibrated refractometer or probe before adding a single drop.
Mixing saltwater properly
Start with pure RO or RO/DI water — tap water carries chlorine, phosphate and metals that undo much of the benefit of the change. Add the salt mix gradually while circulating, rather than dumping it all in at once, so it dissolves evenly instead of settling in a layer at the bottom of the container. Once the mix looks clear and has circulated for several hours, check salinity with the same calibrated instrument you use on the tank, and adjust the temperature to match the display before transferring any water across.
Common mistakes worth avoiding
The biggest mistake is treating a water change as a rescue for a tank that has drifted badly out of range — a single oversized change on a stressed system can do more harm than the problem it was meant to fix, since it forces a rapid parameter shift on top of whatever was already going wrong. A slower correction over several smaller changes is almost always safer. The second common mistake is confusing a water change with a top-off: evaporation should always be replaced with pure fresh water, never saltwater, or salinity will climb steadily over time regardless of how carefully you handle the actual water change.
The last mistake is skipping the test-first step. Testing before a water change tells you whether the change needs to be a routine 10–15% or something more deliberate, and it gives you a baseline to compare against afterwards. Without that baseline, a water change is a guess rather than a measured correction — and the only way to know whether your routine is actually holding parameters steady is to log each change alongside the readings around it and watch the trend, rather than trusting how the tank looks on any single day.
Log every water change in ReefDeck
ReefDeck is a free, offline reef logbook. Note the date, percentage and salinity of every water change, then watch how it lines up with your parameter trends — no account required, and your data stays on your device.
Open ReefDeck — it's free → Works on phone and desktop · installs as an app · exports to CSV anytimeFrequently asked questions
How much water should I change in a reef tank?
A 10–15% weekly water change is a reliable default for most mixed reefs. Heavily stocked or SPS-dominant tanks sometimes prefer smaller, more frequent changes (around 5% twice weekly), while lightly stocked systems with strong skimming can stretch to 20–25% every two weeks.
How often should I do a water change?
Weekly is the most common and easiest cadence to stay consistent with. What matters more than the exact frequency is sticking to a predictable schedule rather than alternating between rushed small changes and infrequent large ones.
Can I change too much water at once?
Yes. A very large water change on a tank that has drifted out of range forces a rapid parameter shift on top of the existing problem, which can stress livestock more than the drift itself. Smaller, more frequent corrections over several days are generally safer than one large change.
Should new saltwater match my tank's temperature and salinity?
Always. Mix new saltwater to the same salinity as your display (about 35 ppt / 1.025–1.026 SG for a mixed reef) and match its temperature before adding it. Mismatched water is one of the most common and most avoidable sources of water-change stress.