How to Do a Water Change on a Reef Tank

What you need

A food-safe bucket or container for new saltwater, a heater and a small pump to mix and warm it, a siphon or hose, a bucket for old water, a refractometer or salinity meter, a thermometer and an old towel. Keep this kit just for the aquarium, so no soap or household chemicals get near it.

Step 1: Mix the new saltwater

Use RODI water, not tap water. Red Sea says to always add salt to water, never the other way round. Follow your salt’s own instructions on temperature and mixing time, because they vary between brands. Red Sea, for example, says to mix its salts in water no warmer than 20 °C, for no more than two hours, then heat it to tank temperature.

Check the salinity of the new water and adjust it with a little more salt or RODI water. Check the tank’s salinity as well, so you’re matching the tank you have, not the number you expect.

Use your real water volume, sump included. The tank volume calculator works it out.

Step 2: Switch off the right things

Turn off the return pump so the sump doesn’t overflow when you refill. Turn off the automatic top-off, or it will try to replace the water you’ve taken out with fresh water. Turn off heaters that will be out of water as the level drops. Many people turn the skimmer off too, so it doesn’t overflow when the level changes.

Leave wavemakers on if they stay underwater, because flow helps lift detritus for the siphon.

Step 3: Take water out where the waste is

Siphon from the places detritus collects: behind rockwork, in corners and from the top layer of sand. Use a turkey baster to blow detritus off rocks first. If you have a sump, the bottom of the sump and the filter section are good places to take water from too.

Take out the volume you mixed. Watch the siphon so you don’t catch snails or small fish.

Step 4: Refill slowly and restart

Pour or pump the new water in slowly, away from corals, ideally into the sump. Then restart everything in reverse: return pump, heaters, skimmer and top-off. Check the heater has come back on and the return pump is flowing normally. Forgetting the top-off or a heater is the most common water change mistake.

Some cloudiness straight after a water change can be a fine precipitate from the new salt. Red Sea says this comes from overheating, over-mixing or too much salt, and is harmless.

Step 5: Note it down

Log the date, how much you changed and anything you noticed, such as a lot of detritus or a filter that needed cleaning. Note the salt and batch if you’ve just started a new bucket. If something changes in the tank a few days later, this is the first thing you’ll want to check.

The Companion can hold the water change and the jobs around it, such as mixing the next batch and changing RODI filters, on one schedule. See how often to change water for choosing an interval.

Can I use tap water for a reef water change?

Red Sea and most reef sources advise against it. Tap water can contain phosphate, nitrate, copper and other things you don’t want. Use RODI water.

How long should new saltwater mix?

It depends on the salt. Follow the maker’s instructions, because they vary. Some say to mix briefly and use soon; others suggest longer.

Does the temperature need to match exactly?

Get it within a degree or so of your tank. A big temperature difference stresses livestock.