Reef Tank Water Change Guide

Jan 18, 2026

Why Water Changes Matter in a Reef Tank

Regular water changes are one of the simplest and most powerful habits you can build for a thriving reef tank. They remove dissolved waste, replenish trace elements, and help stabilize parameters like alkalinity, calcium, and magnesium. Even if you run a strong skimmer and refugium, water changes are your safety net against slow, invisible imbalances.

If you’re just getting started, be sure to read our Beginner Reef Tank Setup guide first so you understand the basics of filtration and cycling before diving into a strict maintenance routine.

How Often and How Much: Finding Your Water Change Schedule

There’s no single perfect schedule, but most reef keepers have success with:

10–15% weekly for mixed reefs and tanks with growing corals
20% every two weeks for lightly stocked or very stable systems
Smaller, more frequent changes (5–10% twice a week) for nano reefs

Let your test kits guide you. Track nitrate, phosphate, alkalinity, calcium, and magnesium. If nutrients creep up or major elements drift between changes, slightly increase the volume or frequency.

Tip: Log your water test results and water change dates. Over a month or two, you’ll see exactly how your tank responds.

Step-by-Step Reef Tank Water Change Process

1. Mix and Prepare Saltwater

Use RO/DI water to avoid adding unwanted nutrients or metals.
Add a quality reef salt mix and stir with a powerhead or pump.
Aim for 1.025–1.026 sg salinity and 24–26°C (75–79°F).
Mix for at least 4–24 hours with a heater and circulation for full dissolution and gas exchange.

For more on stability, see our article on Reef Tank Water Parameters, which covers ideal ranges and how to test them.

2. Prepare the Display Tank

Clean the glass with a scraper or magnet.
Use a turkey baster to blow detritus off rocks and into the water column.
Turn off return pumps and skimmer so the water level stays stable in the sump.

3. Remove Old Water Safely

Use a siphon or pump to remove 10–20% of the total volume.
Vacuum bare-bottom areas or lightly over the sand surface, avoiding deep stirring.
Stop before exposing corals or invertebrates that are mounted low.

4. Add New Saltwater

Double-check temperature and salinity of the new water.
Slowly pump or pour the water into the sump or a low-flow area to avoid blasting corals.
Turn equipment back on and watch for microbubbles or skimmer overflow—you may need to lower skimmer air for an hour.

Tip: Mark your sump with a permanent line for “normal” water level. It makes future water changes and top-off checks much easier.

Common Mistakes and Final Thoughts

Skipping parameter checks: Changing water blindly can mask issues rather than fix them.
Big, infrequent changes: Huge swings (40–50% at once) can shock corals and fish.
Adding unmixed salt directly to the tank: This can burn tissue and create harsh local chemistry spikes.

Consistent, well-planned water changes are less about perfection and more about rhythm. Once you dial in a schedule and method that match your bioload and coral growth, your reef will reward you with better color, polyp extension, and overall stability. Combine this routine with careful feeding and regular testing, and water changes will feel like a quick tune-up rather than a chore.