How to Do a Saltwater Aquarium Water Change: Step-by-Step

To do a water change in a saltwater aquarium: prepare new saltwater at the same salinity and temperature as the tank, switch off the return pump, siphon out 10–20% of the water while vacuuming detritus from the sand, then slowly add the new water and restart your equipment. A routine change takes 20–40 minutes.

Below is the full step-by-step method, the equipment checklist, how to match salinity and temperature, how to siphon sand without making a mess, and the mistakes that most often stress fish and corals. If you want to know how much to change and how often, read how often to change water in a saltwater aquarium.

Updated October 2026 by the team at Natural Seawater Supplier, a Croydon, London-based supplier of live natural seawater for marine aquariums, delivering across the UK 7 days a week.

Key takeaways

• Match new water to the tank within 0.001 specific gravity (about 1 ppt salinity) and within 1°C before adding it.
• Switch off the return pump, auto top-off and any heater that will be exposed before you start draining.
• Hover the siphon 1–2 cm above the sand and clean a different section each week rather than deep-cleaning the whole bed at once.
• Add new water slowly and away from corals; restart the protein skimmer last, 10–15 minutes after everything else.
• Natural seawater skips the 24-hour mixing step: warm it to tank temperature, check salinity and it is ready to use.

Equipment checklist

How to do a saltwater aquarium water change: step by step

1. Test the water first. Record nitrate, phosphate and alkalinity. If nitrate is higher than usual you may want a bigger change this week.
2. Prepare the new water. Synthetic salt must be mixed with RO/DI water and left circulating for around 24 hours. Natural seawater only needs warming: put a heater and small pump in the container a few hours before the change.
3. Match salinity and temperature. Measure both the tank and the new water. Aim for 35 ppt (specific gravity 1.025–1.026) and within 1°C of the tank, usually 24–26°C.
4. Clean the glass and blow off the rocks. Scrape algae first, then use a turkey baster or powerhead to lift detritus off the rockwork so it settles where you can siphon it.
5. Turn off equipment. Switch off the return pump, auto top-off and protein skimmer. Turn off any heater that will sit above the waterline. Display powerheads can stay on unless they will be exposed.
6. Siphon out the old water. Start the siphon and work across the sand, hovering 1–2 cm above the surface. Gather detritus from behind rocks and from the sump. Stop when the bucket marks show you have removed the planned volume.
7. Service the filtration. While the water is low, change filter socks or sponges, empty and wipe the skimmer cup, and replace carbon or phosphate media if due.
8. Add the new water slowly. Pour or pump it into the sump, or onto rock or glass in the display, never directly on corals. For changes over about 15%, take several minutes.
9. Restart equipment. Turn on the return pump, heaters and auto top-off. Wait 10–15 minutes before restarting the skimmer, as fresh water often makes it overflow.
10. Check and record. After an hour, check salinity and temperature again and note the date and volume changed in a log.

How to match temperature and salinity

Salinity is the number most people get wrong. Open-ocean seawater averages about 35 parts per thousand according to NOAA, and that is the standard target for reef tanks. A difference of more than 0.002 SG between new and old water, repeated every week, will make the tank drift.

• Calibrate your refractometer monthly with 35 ppt calibration fluid. RO water only checks zero, not the 35 ppt range you actually use.
• Measure at the same temperature. Readings shift slightly with temperature, so let samples settle before comparing.
• Adjust before you add. If new water is too salty, add a little RO water; if too low, add a little more salt (or more natural seawater).
• Remember evaporation. Top off with fresh RO water to the usual line before you test, or your salinity reading will be high.

Temperature matters most for larger changes. A 10% change at 2°C cooler barely shifts the tank, but a 30% change at 2°C cooler can drop it by half a degree or more in minutes. See our marine tank water parameters guide for the full set of targets.

How to siphon sand in a saltwater tank

Most saltwater sand beds should be cleaned lightly, not dug out. The top layer holds detritus; the deeper layers host bacteria and tiny animals that help process waste.

• Shallow sand (under 2.5 cm): vacuum gently into the surface, one quarter to one third of the bed each week.
• Deep sand bed (over 8 cm): hover above it only. Deep-vacuuming can release trapped hydrogen sulphide and nutrients.
• Bare bottom: siphon detritus straight off the glass every change.
• Pinch the tube if sand rises too far, so it drops back before it reaches the bucket.

Common water change mistakes to avoid

If something has already gone wrong, our water change troubleshooting guide shows which problems a water change fixes and how big it should be.

Natural seawater vs mixing salt for water changes

Our 25-litre food-grade containers make volumes easy to plan: one container is a 10% change for a 250-litre tank, two cover a 200-litre tank at 25%. Typical parameters are 34–36 ppt salinity, 7.5–8.6 dKH alkalinity, 400–450 ppm calcium and 0–0.9 ppm nitrate. We deliver seven days a week across the UK and can even pump water directly into a tank in an upstairs flat; see aquarium saltwater delivery for details.

If you are moving over from synthetic salt, how to switch your reef tank to natural seawater covers the transition. To try it on your next water change, claim a free 25 litres of natural seawater.

Frequently asked questions

How do you do a water change in a saltwater aquarium?

Prepare new saltwater that matches the tank’s salinity and temperature, turn off the return pump, siphon out 10–20% of the water while vacuuming detritus, then slowly add the new water and restart the equipment. Clean the glass first and service the filter socks while the level is low. A routine change takes 20–40 minutes.

Do I need to remove fish during a saltwater water change?

No. Fish and corals should stay in the tank during a normal 10–25% water change. Just keep the siphon away from small fish and invertebrates and add the new water away from corals.

Should new saltwater be the same temperature as the tank?

Yes. Keep new water within 1°C of the tank, which is usually 24–26°C. Small changes are forgiving, but larger changes with cold water can drop the tank temperature quickly and stress livestock.

Can I use tap water for a saltwater water change?

Tap water is not recommended for mixing salt because it can contain phosphate, nitrate, silicate, copper and chlorine or chloramine. Use RO/DI water for synthetic salt, or use natural seawater, which needs no mixing.

Should I vacuum the sand in a saltwater tank?

Yes, but lightly. Hover the siphon just above shallow sand and clean a different section each week, and avoid digging into deep sand beds, which can release trapped nutrients and hydrogen sulphide.

How long should saltwater mix before a water change?

Synthetic salt is usually mixed and circulated for around 24 hours so it fully dissolves and pH and alkalinity settle. Natural seawater does not need mixing; just warm it to tank temperature and check salinity.

Bottom line

A good saltwater water change is simple: matched water, pumps off, siphon detritus as you drain, add new water slowly, restart the skimmer last. Do it the same way every week and it becomes a 30-minute habit. For the bigger picture, read why water changes are important.

Sources

External sources cited in this article (checked October 2026):

• Why is the ocean salty? (NOAA)