Key takeaways
- Always start with RODI water (0 TDS). Tap water in most NZ towns carries nitrate, phosphate, silicate or copper that feed algae and stress corals.
- Mix in a clean, food-grade container with a pump and heater, and let the salt dissolve fully before you use it, usually at least 12–24 hours.
- Aim for a salinity of 35 ppt (about 1.026 SG) at 25–26 °C, and check it with a calibrated refractometer or digital meter, never by eye.
- Match the new water to your tank's temperature and salinity before each water change.
Mixing saltwater is the most repeated job in reef keeping. Whether you are filling a brand-new tank or doing a weekly 10% water change, the quality of the water you mix decides how stable your reef will be. This guide walks you through the exact process we recommend to customers at our Auckland store. It covers the gear, the steps, the target numbers and the common mistakes.
What you need to mix saltwater
You do not need a lot of equipment, but each item matters:
- RODI water: reverse osmosis plus deionisation water with a TDS reading of 0. See our RODI units and filters.
- Quality reef salt: a reef-grade mix with consistent alkalinity, calcium and magnesium. Browse our marine salt range.
- A food-grade mixing container: a dedicated bucket or drum that has never had soap or chemicals in it. A 20–25 litre bucket suits small tanks. Larger reefs often use a 60–200 litre drum.
- A small circulation pump to keep the water moving while the salt dissolves.
- An aquarium heater so the water reaches tank temperature.
- A refractometer or digital salinity meter, calibrated with calibration fluid.
- Test kits to spot-check alkalinity and other parameters on a new batch of salt. See our water testing range.
Why RODI water matters in New Zealand
NZ tap water is safe to drink, but it is not made for corals. Depending on your supply, it can contain chlorine or chloramine, nitrate, phosphate, silicate and traces of copper from household pipes. Even small amounts build up over months of water changes and evaporation top-ups. The result is nuisance algae, diatoms and unhappy corals. Tank water, bore water and rainwater also vary too much to be reliable.
An RODI unit strips these out. Check the output with a TDS meter: anything above 0–1 ppm means it is time to replace your DI resin or filters.
How to mix saltwater step by step
- Fill your container with RODI water. Leave some room at the top so the water doesn't splash as the pump runs.
- Add the pump and heater. Set the heater to the same temperature as your display tank, usually 25–26 °C. Salt dissolves more evenly in moving, warm water.
- Add the salt gradually. Pour it in slowly while the pump runs, rather than dumping it all at once. Most reef salts need roughly 35–40 g per litre to reach 35 ppt, but follow the instructions on your chosen brand and treat that only as a starting point.
- Let it mix. Give it time to dissolve fully and stabilise. Mixing for 12–24 hours is a sensible default. The water should be crystal clear with no residue on the bottom.
- Measure salinity. Use your refractometer or meter at mixing temperature. Add a little more salt to raise salinity, or a little more RODI water to lower it, then let it mix again and re-test.
- Check temperature and, ideally, alkalinity. Testing a new salt batch tells you what you are adding to the tank.
- Do the water change. Remove old water first, then slowly add the new water, avoiding blasting corals or sand.
Target numbers for mixed reef saltwater
| Parameter | Typical reef target | Notes |
|---|---|---|
| Salinity | 35 ppt (≈1.025–1.026 SG) | Match your display tank |
| Temperature | 25–26 °C | Match your display tank |
| Alkalinity | 7–11 dKH | Depends on the salt brand; stability matters most |
| Calcium | 400–450 ppm | Check a new batch of salt |
| Magnesium | 1,250–1,400 ppm | Supports stable alkalinity |
| RODI TDS (before salt) | 0 ppm | Replace DI resin when it rises |
For a deeper look at each parameter, read our reef water chemistry guide.
Choosing a reef salt
Reef salts differ mainly in their alkalinity, calcium and magnesium levels. Some are formulated close to natural seawater. Others are "elevated" to support fast-growing coral reefs. The most important thing is to pick one quality salt and stay with it so your water changes are consistent.
If you are weighing up the two most popular Red Sea mixes, our article Red Sea Reef Salt or Coral Pro: what's best for your reef tank? explains the difference. You can also browse the full Red Sea salt range.
How much saltwater should you mix?
Most reef keepers change 10–20% of their total water volume every one to two weeks. Work out your real water volume, including the sump and minus the rock and sand. Then mix a little more than you need so you can top up after spills. For example, a tank with 300 litres of real water volume needs about 30 litres for a 10% change.
Mixed saltwater can be stored in a sealed, clean container for a short time. Keep a pump running occasionally, or give it a stir and re-check salinity and temperature before use.
Common saltwater mixing mistakes
- Using tap water: the number-one cause of persistent algae in new tanks.
- Not letting the salt dissolve fully: cloudy or freshly mixed water can cause pH and alkalinity swings.
- Measuring salinity with a swing-arm hydrometer: they are cheap but often inaccurate. A calibrated refractometer is far more reliable.
- Topping up evaporation with saltwater: only water evaporates, so top up with fresh RODI water, not saltwater. Otherwise your salinity will creep up.
- Changing salt brands often: each brand has a different ionic balance, so switching often makes your parameters swing.
- Adding water at a different temperature or salinity: this shocks fish, corals and invertebrates.
Get everything you need for perfect saltwater
Shop reef salt, RODI units and filters and water test kits, delivered NZ-wide from our Auckland store. Not sure what you need? Visit us at 114 Wiri Station Road, Wiri, or get in touch and we'll help you choose.
Frequently asked questions
How long should I mix saltwater before using it?
Mix until the water is crystal clear and the salt has fully dissolved, then confirm salinity and temperature. For most salts, 12–24 hours with a pump and heater is a reliable default. Always check your salt brand's instructions.
Can I use tap water to mix saltwater for a reef tank?
It isn't recommended. Tap water can contain chlorine or chloramine, nitrate, phosphate, silicate and metals that fuel algae and harm corals. Use RODI water with a TDS of 0.
What salinity should a reef tank be?
Most reef tanks are kept at 35 ppt, which is about 1.025–1.026 specific gravity. Stability is more important than chasing an exact number.
Should I add salt to the water, or water to the salt?
Add salt to water. Start with your measured RODI water, get it moving and heating, then add the salt gradually.
