Reef Tank Water Change Calculator

A reef tank water change calculator works out exactly how many gallons of saltwater to mix and how much salt mix you need for a single water change. Enter tank size, sump volume, and a target percentage to get precise amounts in gallons or liters.

Track every water change and parameter trend with My Reef Log - free reef keeping journal.Start tracking free →

Tank Volume

Enter your display and sump capacity

Water Change Percentage

20%

Most reefers change 10-25% every 1-2 weeks

5%20%50%

Salt Mix & Salinity

Most salt mixes yield about 0.45 gal of saltwater per pound at 1.025 SG

Schedule Cadence

How often you change water - drives annual cost and salt usage

Water Change Plan

95 gal
360 L
Gross volume
85.5 gal
324 L
Net water (-10%)
20%
SG 1.025
Change %
Every 2 weeks
26x / year
Cadence
Saltwater to mix per change
17.1 gal
(64.7 L)
Mix RO/DI water with 38 lb (17.2 kg) of salt mix to reach SG 1.025.
Cost per change
$57.00
38 lb × $1.50 / lb
Salt per change
38 lb
17.2 kg
Annual (every 2 weeks)
Saltwater per year
445 gal
1,683 L
Salt per year
988 lb
448 kg
Salt cost per year
$1,482.00
excludes RO/DI water

Water Change Tips

  • ✓Match temperature and salinity. Mix new saltwater the day before, aerate it overnight, and confirm salinity matches your display tank within ±0.001 SG before adding.
  • ✓Use RO/DI water only. Tap water introduces phosphates, silicates, copper, and chlorine that fuel algae and harm corals. A 0 TDS RO/DI source is non-negotiable for reefs.
  • ✓Smaller, more frequent beats large and rare. Two 10% weekly changes are gentler on livestock than a single 20% monthly swap because parameter swings stay smaller.
  • ✓Stir the sand and blow off rocks first. Detritus suspended in the water column gets siphoned out with the change - waiting until after the change leaves it to settle right back.
  • ✓Log every change. Tracking date, percentage, and parameters before/after reveals whether your routine is actually keeping nitrate, phosphate, and alkalinity stable over time.

How It Works

  1. 1Sum display + sump volume. The calculator adds display and sump volume, then subtracts your rock/sand displacement percentage to get net water volume.
  2. 2Apply your change percentage. Net volume multiplied by your chosen percentage (5-50%) gives the exact volume of saltwater to mix.
  3. 3Convert to salt mix amount. Saltwater gallons divided by your salt mix yield (default 0.45 gal/lb) gives pounds of salt to scoop, then converted to kilograms.
  4. 4Project annual usage and cost. Pick weekly, biweekly, or monthly cadence to see how much saltwater, salt, and money you'll spend per year on water changes.

Frequently Asked Questions

How much water should I change in a reef tank?

Most reef keepers change 10-25% of their net water volume every 1-2 weeks. Smaller, more frequent changes (10% weekly) keep parameters more stable than larger monthly changes. For a 75 gallon system with a 20 gallon sump (about 85 gallons net), a 20% change is roughly 17 gallons of fresh saltwater.

How often should I do a reef tank water change?

Weekly or biweekly is the sweet spot for most reef tanks. Frequent small changes export nutrients gradually and avoid the parameter swings of large monthly changes. Heavily stocked SPS tanks benefit from weekly 10-15% changes, while lightly stocked LPS or mixed reefs do well on biweekly 20% changes.

How do I calculate true tank volume with rock and sand?

Add display gallons plus sump gallons, then subtract roughly 10% for live rock and sand displacement. A 75 gallon display with a 20 gallon sump is 95 gallons gross but about 85 gallons of actual water. Heavily aquascaped tanks may need a 12-15% deduction.

How much salt mix do I need per gallon of saltwater?

Most reef salt mixes yield about 0.45 gallons of saltwater per pound at 1.025 specific gravity. So 10 gallons of saltwater needs roughly 5.5 cups or about 4.5 pounds of dry salt. Always check your specific brand's instructions and confirm with a refractometer.

What salinity should reef tank water changes target?

Target 1.025 specific gravity (35 ppt) for reef tanks - this matches natural seawater and is what corals evolved in. Mix new saltwater to match your display tank exactly within ±0.001 SG, and verify with a refractometer rather than a swing-arm hydrometer for accuracy.

Related Reef Tools

Salt Mix Calculator

Brand-specific salt amounts for any tank volume

Aquarium Volume Calculator

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Water Parameter Calculator

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Dosing Calculator

Calculate supplement dosing amounts

Track Every Water Change

Logging water changes alongside parameter tests reveals whether your routine is actually keeping nitrate, phosphate, and alkalinity stable. My Reef Log is a free reef keeping journal that tracks it all in one place.

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