Water changes: how often, how much, and how to make them painless

The standard answer

10–20% of tank volume, every 1–2 weeks. That's the textbook recommendation. It's also fine for the vast majority of reef tanks.

What water changes actually do

1. Replenish trace elements — calcium, magnesium, iodine, etc. that get consumed
2. Export accumulated waste — nitrates, phosphates, organic byproducts that build up
3. Stabilize alkalinity — fresh saltwater resets buffering capacity

If you're heavy-dosing two-part or running a calcium reactor, water changes become more about #2 and less about #1.

Doing it right

1. Mix saltwater 24+ hours ahead. Salt needs to fully dissolve and pH/alk needs to stabilize. Mixing fresh and dumping in immediately stresses livestock.
2. Match temperature. Within 1°F of tank temp at the moment of replacement.
3. Match salinity. Test the new water with a refractometer before adding.
4. Don't blast the rockwork. Drain slowly to keep detritus suspended for export, then refill against the side glass to avoid disturbing the aquascape.

When water changes aren't enough

If you're seeing nitrate climb above 20 ppm or phosphate above 0.1 ppm despite weekly water changes, the system is overstocked or overfed. Water changes can't keep up with input. Address the source: feed less, stock less, or add nutrient export (skimmer, refugium, GFO, biopellets).

The "no water change" debate

Some advanced reefers run no-water-change systems with extensive automation. This works only with mature systems, comprehensive testing, and disciplined dosing. It is not a beginner approach. For your first 1–2 years, do water changes.