What Water Changes Actually Do

Understanding what water changes accomplish makes the routine make sense, and helps you diagnose when something else is needed alongside them.

How Much and How Often, Actual Numbers

The Standard Recommendation

For most beginner reef tanks: 10–15% of total system volume
every two weeks. This is the maintenance baseline, the schedule
that keeps a moderately stocked tank in good nutrient balance with
consistent execution.

“Total system volume” means display plus sump (if present). A 30-gallon
display with a 10-gallon sump is a 40-gallon system, a 10% water change
is 4 gallons, not 3.

Volume Reference by Tank Size

When to Increase Frequency

• Active algae outbreak, increase to 10–15% twice per week until nitrate and phosphate return to target range, then resume bi-weekly
• After a disease treatment, many medications need to be removed after treatment; frequent water changes accelerate removal
• After a parameter crash, ammonia spike, alkalinity precipitation, salinity accident, increased water changes help reset chemistry faster
• New tank, first 3 months, more frequent small changes help manage the higher nutrient variability of a maturing biological system
• Heavily stocked tank, more bioload means faster nitrate accumulation; frequency should match the tank’s actual production rate, not a fixed calendar

Why Consistency Beats Volume

The math favors regular small changes over occasional large ones. A 10%
change removes 10% of the nitrate present at that moment. A second 10%
change two weeks later removes 10% of the remaining nitrate, and the
tank has had two weeks of continued biological filtration adding to it.
Letting nitrate accumulate for six weeks and then doing a 40% change
removes a larger portion in one event, but the livestock spent six
weeks in elevated nitrate conditions, and the large change is more
disruptive to chemistry than two smaller ones would have been.

The tank that’s changed consistently is always near its best water quality.
The tank that’s changed occasionally spends most of its time in a degraded
state with periodic resets.

Salt Mix, What to Choose and Why It Matters

Not all reef salt mixes are equivalent. The differences that matter for
reef tanks are alkalinity, calcium, and magnesium levels in the mixed
solution, and the consistency of those levels batch-to-batch.

What to Look for in a Reef Salt

• Alkalinity at mix: 8–10 dKH at 1.025 specific gravity
• Calcium at mix: 400–450 ppm
• Magnesium at mix: 1280–1350 ppm
• Consistent batch-to-batch results, cheap salt mixes vary significantly between bags; quality brands test consistently
• Full trace element profile, iodine, strontium, molybdenum, and other minor elements should be listed

Recommended Reef Salt Mixes

Important: Always test the alkalinity and calcium of
your freshly mixed saltwater before adding it to the tank. Different
salt mixes produce different parameter levels, and those levels vary
depending on how much salt you add per gallon. Knowing what your mixed
water measures prevents compounding an existing parameter problem with
every water change.

Setting Up a Mixing Station

A mixing station is simply a designated area where you prepare and store
saltwater before it goes into the tank. It doesn’t need to be elaborate, a 20-gallon Brute trash can, a small powerhead, and a dedicated heater
is a complete mixing station for most home reef tanks.

What You Need

• Mixing container, a food-safe plastic bin or Brute trash can sized to hold at least one full water change volume. Dedicated to saltwater use only, never used for anything that contained soap, chemicals, or cleaners.
• Small powerhead or circulation pump, to dissolve salt and circulate the mix. A powerhead running continuously produces a fully dissolved, homogenous mix.
• Dedicated heater, to bring new water to tank temperature before use. A $15 submersible heater set to 78°F is sufficient.
• Refractometer or salinity probe, to verify salinity before adding water to the tank. The single measurement that matters most at change time.
• RODI water source, via a home RODI unit, purchased RODI water, or a local fish store that sells RODI. Never tap water.

How Long to Mix Before Use

Mix saltwater at least 24 hours before use, 48 hours
is better. Fresh salt mix that hasn’t fully dissolved can have elevated
pH and inconsistent parameters that stress coral tissue if added to the
display before the chemistry has equilibrated. Saltwater that’s been
mixed and circulating for 24+ hours is chemically stable, fully oxygenated,
and at a consistent parameter baseline. Some reefers mix a week’s supply
at once and store it ready to use, this is fine as long as the container
is covered and the powerhead keeps it circulating.

The Water Change Procedure, Step by Step

Before You Start

• Confirm new saltwater has been mixed for at least 24 hours
• Test new water salinity, should match tank within 0.001 specific gravity (1.024–1.026)
• Test new water temperature, should match tank within 1°F
• Have a siphon hose, bucket or drain line, and the new water container ready

Step 1, Turn Off Equipment

Before removing water, turn off:

• Return pump, prevents running dry if sump water level drops significantly
• Protein skimmer, skimmers behave erratically when water level drops; prevents skimmer from pulling air and flooding the collection cup
• Auto top-off system, ATO will try to compensate for the water you’re removing by adding fresh RODI, turn it off so it doesn’t refill while you’re working
• UV sterilizer if present, can run dry if plumbed through sump

Leave wavemakers running in the display if possible, flow keeps detritus
in suspension so the siphon can capture it.

Step 2, Remove Old Water and Siphon Detritus

Use a siphon hose to remove the target volume of water. This is also the
most effective time to remove accumulated detritus, waste that’s settled
in low-flow areas of the sandbed, in corners, and around the base of rockwork.

Detritus Siphoning Technique

Hover the siphon end 1–2 inches above the sandbed surface in detritus-heavy
areas, don’t push it into the sand. Water movement from the siphon flow
draws loose detritus up into the hose without disturbing the sand structure
or releasing hydrogen sulfide from deeper anaerobic zones.

Work systematically: start at one end of the tank, move slowly across
visible detritus patches, and finish at the corner where your siphon exits.
In a tank with an active clean-up crew and good flow, most detritus is already
processed, you’ll see very little to siphon. Significant visible detritus
is a flow or CUC issue worth investigating.

Siphon from the sump (if present), the return section of the sump accumulates
fine detritus that passed through mechanical filtration. A brief siphon of
the sump bottom removes this before it decomposes and adds to nitrate.

Step 3, Match Salinity and Temperature

Before adding new water, confirm both match. This is the step most beginners
skip and the one that causes livestock stress when skipped.

• Salinity mismatch: New water more than 0.001–0.002
  specific gravity from tank salinity causes osmotic stress, particularly
  for invertebrates and corals. If the new water is significantly higher
  or lower, adjust by adding RODI water (to lower) or dry salt (to raise)
  and retest before use.
• Temperature mismatch: New water more than 2°F from tank
  temperature causes a thermal shock response. If the mixing heater hasn’t
  brought the new water to temperature yet, wait, or add the water extremely
  slowly to allow it to equilibrate without a sharp local temperature drop.

Step 4, Add New Water Slowly

Pour new saltwater into the sump return section (if present) or directly
into the display. Slow addition, over 5–10 minutes rather than all at once, prevents local salinity and temperature shocks even when the parameters are
well-matched. If adding to the display directly, pour onto a plate or flat
surface to diffuse the flow and avoid blasting the sandbed.

For larger tanks (50+ gallons) where the volume of new water is significant,
consider using a small pump to transfer water from the mixing container at
a controlled rate rather than pouring by bucket.

Step 5, Restart Equipment and Verify

1. Restart the return pump first, verify flow is normal
2. Restart the protein skimmer, expect temporary erratic behavior for 15–30 minutes as it adjusts to the new water chemistry; this is normal
3. Re-enable the auto top-off system
4. Check water level in the sump, should be at normal operating level
5. Verify temperature is holding at target
6. Take a final salinity reading, confirm the display is still at 1.025–1.026

Step 6, Record the Change

Note the date, volume changed, and any observations (parameters tested before
the change, detritus volume observed, any livestock behavior worth noting).
A water change log takes 30 seconds to update and turns your maintenance
history into a diagnostic tool, when something goes wrong three months from
now, knowing whether it coincides with a missed change or a parameter drift
is useful information.

What to Test Before and After Water Changes

Test before the water change, not after. Post-change readings
reflect the dilution of the change, not the tank’s actual stable baseline.
Testing before gives you the real status of what the tank was maintaining
between changes.

👉 Reef Tank Water Testing Guide, Full Parameter Reference

When Water Changes Aren’t Enough

If nitrate or phosphate continue rising despite consistent bi-weekly
water changes, the nutrient input is exceeding what the changes can export.
Water changes slow the rise but can’t reverse it if the source is producing
faster than the changes remove. Diagnose the source:

• Overfeeding, the most common cause. Reduce feeding
  volume. Strain frozen food liquid. Skip one day per week.
• Old filter floss, floss more than 5–7 days old is
  decomposing and releasing nutrients. Replace every 5 days without exception.
• Skimmer underperforming, clean the neck, verify operating
  water depth, check that it’s producing dark skimmate. A skimmer running
  wet or producing light foam is removing a fraction of what it should.
• Dead animal, a fish or invertebrate decomposing
  undetected adds a continuous organic load. Count all fish daily.
• Tap water use, if using tap water for top-offs or
  water changes, every gallon adds nitrate and phosphate. Switch to RODI.
• Overstocked tank, more fish than the filtration
  system can process. Evaluate bioload relative to tank volume and
  filtration capacity.

If nutrient sources are addressed and nitrate is still rising, add a
refugium with chaeto for continuous biological export, or increase
water change volume temporarily to 20% weekly until parameters trend down.

Common Water Change Mistakes

Water Change Quick Reference