Setup path, stage 4

RO/DI water and mixing reef salt for a new tank

Make clean water, mix it to a salinity you can measure and hold, and get the temperature steady. Everything later depends on these three numbers being right.

Last checked 21 October 2026. How we check

What do you need for this stage?

• RO/DI unit with a TDS meter, or RO water from a shop
• Reef salt mix
• Refractometer and 35 ppt calibration fluid
• A food-safe mixing container, heater and pump
• Two thermometers

Do I need RO/DI water in the UK?

Yes for almost everyone. BRS says water for top-off and mixing salt should read 0 TDS. Much UK tap water is treated with chlorine or chloramine and can carry phosphate and silicate, which feed algae. Check your supplier's water quality report for your postcode if you want to know what yours carries.

Typical upkeep, from keepers rather than makers (typical, check): change the DI resin when the meter after it reads above 0, change the sediment and carbon prefilters about every six months, and replace the membrane when its output TDS passes about 10% of the tap reading. Never plumb RO/DI straight into the top-off reservoir without a float valve and a shut-off (BRS).

How do I mix salt water?

1. Fill the container with the full volume of RO/DI water and heat it to 23 to 25 C with a pump running.
2. Add salt to the water, not water to salt. Tropic Marin warns that strong mixes can precipitate calcium carbonate.
3. Start with the weight on the bag, or the salt mix calculator. Brands differ: retailer copies of labels give roughly 33 to 40 grams per litre depending on brand and target (typical, check).
4. Mix until clear and the reading is steady. Keepers usually mix for two to three hours, not a day (typical).
5. Measure with a calibrated refractometer at about 25 C and adjust with small amounts of salt or RO/DI. The salinity correction calculator works out how much.

What salinity and temperature should a reef tank be?

Most sources centre on 35 ppt, which is a specific gravity of about 1.026 at 25 C. Charterhouse gives 1.025 to 1.026. Red Sea sets 33 to 35 ppt depending on tank type. Pick one figure and hold it: the steadiness matters more than which of these you pick.

Calibrate the refractometer with 35 ppt seawater standard, not RO water. Retailers report that one calibrated with plain brine can read seawater up to about 1.5 ppt out (typical).

Charterhouse suggests 24 to 26 C. We suggest 25 to 26 C, held within half a degree each day. Always fix salinity before you test anything else: Red Sea notes that a 1 ppt rise from evaporation lifts calcium by about 13 ppm and magnesium by about 40 ppm.

What are the steps, in order?

1. Set up the RO/DI unit with a drain and a float valve on any storage container.
2. Check the TDS meter reads 0 after the DI stage.
3. Mix enough salt water to fill the system, plus a spare batch for emergencies.
4. Heat and circulate, then measure salinity with a calibrated refractometer.
5. Fill the tank, start the heaters and pumps, and check temperature against a second thermometer.
6. Recheck salinity and temperature after 24 hours.

Your checklist

Tick items as you go. Ticks are saved in this browser only and fill the depth gauge.

What test must pass before moving on?

What are the common mistakes?

• Mixing with tap water.
• Calibrating the refractometer with RO water.
• Pouring water onto a heap of salt, which can leave a cloudy precipitate.
• Testing alkalinity or calcium before fixing salinity.
• Trusting one thermometer, often a stuck one.

Where do sources disagree?

Figures from sources read as a search summary only are shown as typical figures: check them before you rely on them. Found a mistake? Tell us how to fix it.