Reef Tank Water Changes: What Actually Works

Water Changes: The Foundation of Reef Keeping

Getting a water change routine dialled in is one of the most impactful things for a tank's long-term health. Not glamorous, but undeniably effective.

How Often and How Much?

The classic advice is 10-20% weekly, but the real answer depends on the setup:

Soft coral / fish-only tanks: 15-20% every week or two is usually plenty. These systems are more forgiving.
Mixed reef with LPS: 10-15% weekly keeps things stable.
SPS-dominant tanks: Some keepers do 10% weekly, others rely on dosing and barely change water at all.

The "Do Water Changes Even Matter?" Debate

There's a school of thought that says proper dosing, a good skimmer, and clean ICP tests mean water changes can be reduced or skipped. Some experienced reefers do exactly that with great results.

However, water changes do things dosing can't — they dilute dissolved organics, trace element imbalances, and compounds that test kits don't even measure. Think of it as a reset button for the things that can't be seen.

Getting the Basics Right

Salt mix matters. Use a reef-grade salt with elevated calcium and alkalinity. Mix it the night before in a clean bucket with a small powerhead and a heater. Target 1.025-1.026 specific gravity.

Temperature match. Get new water within 1°C of tank water before adding it.

RODI water only. Tap water introduces phosphate, silicate, chloramine, and heavy metals. An RODI unit pays for itself within months.

A Simple Process

Turn off the return pump (leave wavemakers running)
Siphon old water from the sump or rear chamber — vacuum any detritus at the same time
Clean the glass with a magnetic scraper while the water level is low
Slowly add new saltwater
Turn the return pump back on
Test salinity, done

Tip: Siphon from the bottom of the sump, not the display. That's where gunk accumulates.

When to Change More

During algae blooms, nutrient spikes, or a generally struggling tank, bigger water changes can help. Two 25% changes a few days apart is safer than one 50% change to avoid parameter shock.

What's your water change routine? Weekly, fortnightly, or a low/no water change system? What's working for different setups?