BRIEFING

Mixing Saltwater the Right Way: A Reef Water Chemistry Guide

May 27, 2026 · Maren Marine
Mixing Saltwater the Right Way: A Reef Water Chemistry Guide

Every reef starts with water you make yourself — and getting that water right is the most underrated skill in the hobby. Nail it and half your problems never happen. Here's how to source it, mix it, and measure it without overcomplicating things.

Start upstream: your water source

Tap water carries chlorine, nitrate, phosphate, silicates and sometimes copper — fuel for algae and trouble for inverts. That's why serious reefers don't mix salt into tap. They start with RO/DI water: reverse-osmosis plus deionization strips it back to a near-zero blank slate, so the only things in your tank are the ones you put there.

Straight talk — If you take one thing from this guide: an RO/DI unit is the highest-return purchase a new reefer can make. Skipping it is the single most common root cause of stubborn algae — and you'll spend more fighting that than the unit costs.

RO/DI systems

BRS 4-Stage 75 GPD RO/DI · $160
The standard, proven starting unit. Produces clean 0 TDS water for mixing and top-off.

SpectraPure MaxCap RO/DI w/ Booster · $300
Higher rejection and a booster pump for low-pressure homes — the upgrade when you want maximum purity and faster output.

TDS Pen · $15
A two-dollar habit that saves tanks: check your RO/DI output reads 0 TDS so you know your filters are still doing their job.

Pick a salt, then stay with it

Reef salts are formulated to hit reef parameters when mixed to the right salinity. The differences between good brands are small; consistency matters more than brand. Pick a reputable mix and stick with it so every water change lands in the same place.

Salt mixes

Red Sea Coral Pro · $70
Slightly elevated alk/cal/mag — a popular, forgiving choice for growing tanks.

Tropic Marin Pro-Reef · $100
Premium, ultra-consistent batches prized by SPS keepers who want zero surprises.

Fritz RPM Reef Pro Mix · $63
Great value, dissolves fast and clear — an easy everyday mix.

Mixing, step by step

  1. Fill a clean container with RO/DI water (never add salt to tank water).
  2. Add a powerhead and a heater — mix at tank temperature.
  3. Add salt gradually while the water circulates. Don't dump it all at once.
  4. Let it mix and aerate for a few hours to overnight before testing or using.
  5. Measure salinity and adjust to 1.025 specific gravity (≈35 ppt) — add water to lower, add salt to raise.
Reefer's tip — Measure salinity with a refractometer or a quality digital meter, not a cheap swing-arm hydrometer — those drift badly and have wrecked more parameters than they've saved. And keep premixed saltwater on hand so water changes are a five-minute job, not an afternoon project.

The numbers that actually matter

You don't need a chemistry degree — you need to watch a short list and keep it steady:

Target ranges
  • Salinity — 1.025 sg / 35 ppt
  • Temperature — 76–78°F, stable
  • Alkalinity — 8–9 dKH (the big one — stability beats a perfect number)
  • Calcium — ~420 ppm · Magnesium — ~1350 ppm
  • Nitrate — a little is fine (2–10 ppm); zero starves coral · Phosphate — ~0.03–0.1 ppm

Testing

Red Sea Reef Foundation Test Kit · $45
Track the big three (alk, calcium, magnesium) at home — your everyday dashboard.

ICP Water Analysis (Mail-In) · $40
A few times a year, send a sample for a full breakdown of trace elements and contaminants your home kit can't see.

Find everything in Salt Mix, ATO & RO/DI and Test Kits.

Setting up from scratch? The Build Studio includes the RO/DI, salt and testing your tank needs — sized and bundled so nothing's missing.

Open the Build Studio →

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