How to Mix Saltwater for Your Reef Tank: A Step-by-Step Guide

Why Mixing Your Own Saltwater Matters

The water in your reef tank is the foundation of everything. Get it right and your corals, fish, and invertebrates will thrive. Get it wrong and you'll be chasing problems indefinitely. Mixing your own saltwater from scratch — using pure RODI water and a quality dry salt mix — gives you full control over what goes into your tank and is far more economical than buying pre-mixed water long term.

This guide walks you through exactly how to do it, from the equipment you need to the step-by-step mixing process.

What You'll Need

  • A RODI unit — to produce pure, 0 TDS source water
  • A dry reef salt mix — your choice of brand
  • A clean mixing container — a dedicated food-grade bucket or bin (never use one that's had chemicals in it)
  • A powerhead or small pump — to circulate and dissolve the salt
  • A heater — to bring the water to tank temperature before adding it
  • A refractometer — to measure salinity accurately

Step 1 — Make Your RODI Water

Never mix salt with tap water. Tap water contains chlorine, chloramines, heavy metals, and silicates that can cause algae blooms, harm livestock, and interfere with your tank's chemistry. Always start with pure RODI (Reverse Osmosis Deionised) water.

Our 4 Stage 75GPD RODI System produces high-purity water at 0 TDS, giving you the perfect blank canvas for mixing reef-quality saltwater at home. A home RODI unit pays for itself quickly compared to buying pre-made water, and you'll always have fresh water on demand.

Fill your mixing container with the volume of RODI water you need and allow it to reach room temperature before adding salt.

Step 2 — Choose Your Salt Mix

Not all salt mixes are created equal. For a reef tank, you want a mix that dissolves clearly, has consistent batch-to-batch chemistry, and is formulated for the elevated calcium, alkalinity, and magnesium levels that corals need. We stock a range of trusted options:

 

As a general rule: use a standard reef salt for mixed or LPS tanks, and an SPS-specific salt if you're running a high-demand SPS system.

Step 3 — Add Salt and Mix

Place your powerhead or small pump in the mixing container to circulate the water, then add your salt gradually. Most salt mixes dissolve at approximately 37–40g per litre to reach a salinity of 35 ppt (1.025–1.026 SG) — but always follow the manufacturer's instructions on the bag, as ratios can vary slightly between brands.

Run the pump for at least 30–60 minutes to ensure the salt is fully dissolved. The water should be completely clear with no visible undissolved particles before you test or use it.

Step 4 — Heat to Tank Temperature

Add a heater to your mixing container and bring the water to your tank's temperature (25–26°C) before adding it to your aquarium. Adding cold water to a warm tank stresses livestock and can cause temperature swings that affect coral health.

Step 5 — Test Salinity Accurately

This is the most important step. Salinity must be measured accurately — a cheap plastic hydrometer is not good enough for a reef tank. Use a quality refractometer calibrated with RODI water.

We stock several reliable options:

Your target salinity is 1.025–1.026 SG (35 ppt). If it's too high, add a small amount of RODI water and retest. If it's too low, dissolve a small additional amount of salt separately and add it to the mix.

Step 6 — Add to Your Tank

Once your new saltwater is at the correct temperature and salinity, you're ready to add it to your tank. For water changes, remove the old water first, then slowly add the new water to avoid disturbing your sandbed or stressing livestock. A slow drip or gentle pour works well.

How Often Should You Do Water Changes?

For most reef tanks, a water change of 10–20% every 1–2 weeks is a good baseline. Water changes replenish trace elements, export nitrate and phosphate, and help keep parameters stable. In heavily stocked or SPS-dominant tanks, more frequent changes may be needed.

Tips for Consistent Results

  • Always calibrate your refractometer with RODI water before each use — temperature affects readings.
  • Mix salt at least 24 hours in advance when possible. Freshly mixed saltwater can have slightly elevated pH and unstable parameters that settle after a day.
  • Label your mixing container as reef use only and never use it for anything else.
  • Check your RODI membrane and DI resin regularly. A TDS meter on the output will tell you when it's time to replace your resin — anything above 0 TDS means your water isn't pure.

Ready to Stock Your Tank?

With your water dialled in, you're ready to start adding livestock. If you're just getting started, check out our guides on how to cycle a saltwater tank and zoanthid and soft coral care. When you're ready to shop, browse our full Corals collection.

Questions about water chemistry or mixing? Get in touch — we're always happy to help.

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