How Often Should You Do Water Changes in a Reef Tank?

The standard answer — 10% weekly — is correct for most reef tanks, and it's worth understanding why before you adopt it, because the reasoning tells you when to deviate. Water changes are the one maintenance act that does four jobs at once, and your ideal schedule depends on which of those jobs your particular tank needs most.

What a water change actually does

1. Exports what accumulates. Nitrate, phosphate, and dissolved organics build continuously from feeding and waste. Removing 10% of the water removes 10% of everything dissolved in it.
2. Replenishes what depletes. Corals consume alkalinity, calcium, magnesium, and trace elements. Fresh salt mix arrives with all of them at target levels — for lightly stocked tanks, water changes alone can be the dosing system.
3. Resets slow drift. Dozens of parameters you never test (trace metals, organics) creep over months. Regular changes keep the tank tethered to the known chemistry of your salt mix.
4. Forces you to look. The most underrated function. The keeper with a weekly water change habit notices the hiding fish, the receding coral, and the failing pump a week before the keeper without one.

Notice what's not on the list: water changes are not primarily an emergency ammonia tool (a cycled tank handles ammonia), and they're not a cure for chronic problems whose input rates exceed the export math — more on that below.

The schedule, by tank type

The default: 10% weekly

Right for the classic beginner setup — a 20–40 gallon mixed reef, a few fish, softies and LPS. Weekly 10% keeps nutrients bounded, tops up alkalinity and calcium consumption at beginner coral loads, and builds the observation habit. In a 30-gallon tank that's about 3 gallons: one bucket, fifteen minutes.

The equivalent: 20% biweekly

Mathematically similar export over a month, and fine for stable, lightly stocked tanks or busy schedules. You trade a little stability for convenience — parameters sawtooth slightly more between changes. If your tank runs a skimmer and modest bioload, you'll never see the difference. If it's a skimmerless nano, stay weekly.

Nano tanks (under 15 gallons): 10–15% weekly, non-negotiable

Small water volumes drift fast — evaporation concentrates salinity, one overfeed spikes nutrients, and there's no dilution buffer. Weekly changes are the nano keeper's primary life-support system. The upside: 1–2 gallons is trivial to mix and carry.

Heavily stocked or heavily fed tanks: 15–20% weekly

A big fish population or generous feeding raises the input side of the ledger; raise the export side to match. Let your nitrate and phosphate tests make this call — if nitrate climbs week over week at 10%, the tank is telling you the percentage is too low (or the feeding too high).

Mature SPS systems: often less, because dosing takes over

You'll meet advanced keepers changing 5% monthly or barely at all, and it confuses beginners. Context: those tanks run dosing systems for alk/calcium, heavy skimming and media for export, and periodic lab testing to catch drift. They've replaced the water change's four jobs with dedicated equipment for each. That's a valid graduate path — not a shortcut available on day one.

How to do a water change properly (the 15-minute version)

1. Mix new saltwater in advance — RO/DI water plus a quality reef salt, mixed with a small pump or powerhead until fully dissolved, ideally heated and circulating for a few hours or overnight.
2. Match salinity exactly to the tank — verify with a calibrated refractometer, not a swing-arm hydrometer. Chasing clarity with mismatched salinity trades one problem for a worse one.
3. Match temperature within a couple of degrees.
4. Siphon out tank water — and make it count: hover the siphon over the sandbed to lift detritus, and pick a different rock's crevices to blast each week with a turkey baster before you siphon.
5. Refill gently — pour against a rock or your hand so the sandbed stays put, or you'll spend a day on the cloudy water this causes.

Two habits that upgrade the routine: clean the glass before siphoning (so the scraped film exits with the old water), and never mix salt directly in the tank — undissolved salt is caustic to whatever it lands on.

When the schedule should flex

Change extra water after events: a death you found late, an overdose of anything, a mini-cycle, visible pollution. For genuine emergencies (ammonia in a stocked tank, chemical contamination), 30–50% changes with well-matched water are safe and appropriate — matched salinity and temperature are what make big changes safe.

Don't scale up to chase chronic numbers. Here's the math that surprises everyone: a 10% change removes 10% of the nitrate once. If nitrate is 40 ppm and rising weekly, water changes alone are a treadmill — the fix is the input side (feeding, stocking, export equipment). Chasing chronic nutrients with heroic water changes is expensive, laborious, and loses to arithmetic every time.

Skip weeks rarely, never twice in a row. Life happens. One missed week in a healthy tank is nothing. Two becomes three becomes the drifted, algae-ridden tank that "suddenly" has problems.

The water change is the anchor of the broader weekly routine — glass, film, top-off, tests, observation — that prevents 90% of reef disasters. The full 30-minute liturgy is laid out in our beginner's guide, The First Tank.

FAQ

Can I do too many water changes?

Daily small changes with well-matched water won't hurt anything — but past a point they're effort without benefit, and large frequent changes can keep a tank chemically unsettled, especially if salinity or alkalinity of the new water doesn't quite match. If your routine exceeds ~20% weekly on a healthy tank, something upstream (feeding, stocking, filtration) deserves the attention instead.

Do water changes remove the beneficial bacteria?

No — nitrifying bacteria live on rock, sand, and surfaces, not in the water column. You can change 50% of the water without denting the biofilter. (Cleaning all the rock and sand at once is a different story; disturb no more than a patch at a time.)

Should I vacuum the sand every water change?

Lightly, in patches, yes — hover the siphon to lift detritus without deep-plowing the bed. Deep-vacuuming an established sandbed all at once can release accumulated hydrogen sulfide and nutrients; rotate zones instead, one region per week.