Setup
How do I mix saltwater to the right salinity?
beginner
4 min read
Water chemistryEquipment
When: Every time you prepare new saltwater — first fill and every water change after
Short answer: Dissolve a reef salt mix into RO/DI water until it reads 1.025 sg (35 ppt) on a refractometer, then let it circulate with a pump or powerhead for a few hours (many reefers go overnight) so it fully dissolves and the temperature settles — don't pour freshly-mixed water straight into the display.
The details
- Follow the bag, then test. Salt-to-water ratios vary by brand — use the manufacturer's starting ratio, then verify with a test; never trust a scoop count alone.
- Use a refractometer, not a swing-arm hydrometer. Swing-arm hydrometers are a classic beginner-accuracy trap; a refractometer (calibrated against a reference fluid) — or a hand-blown glass hydrometer verified against a refractometer — is the standard this corpus points to.
- Mix in a separate container, not the display. Circulate it with a powerhead or pump until the salt is fully dissolved and any cloudiness clears, and let temperature match the tank before it goes in.
- 1.025 sg / 35 ppt is the standard reef target cited across water-chemistry sources. 1.024-1.026 sg is the acceptable window the cycling research cites for verifying stability before livestock — the same range is a reasonable target while you're stabilizing an empty setup-stage tank.
The numbers
| Parameter | Target |
|---|---|
| Salinity | 1.025 sg / 35 ppt |
| Acceptable range | 1.024-1.026 sg |
| Tool | Refractometer (calibrated) |
| Mix/circulation time before use | Several hours to overnight |
Common mistakes
- Trusting a cheap swing-arm hydrometer instead of a refractometer — one of the most common sources of beginner parameter drift.
- Adding fresh-mixed saltwater before it's fully dissolved and temperature-matched.
- Eyeballing the ratio instead of testing every batch — different salt brands, and even different buckets of the same brand, can vary.
When to worry
- Normal: Minor day-to-day drift from evaporation between top-offs — top off with fresh RO/DI water (or automate it later with an auto top-off/ATO) to hold salinity steady.
- Worry: Salinity outside 1.024-1.026 sg, especially right before this stage's gate — the setup stage isn't done until salinity holds ~1.025 sg for a few days with everything running.
What's next
With salinity dialed in, figure out what else the tank actually needs on day one in what-equipment-do-i-need-to-start.
Target parameters
salinity_target1.025 sg / 35 ppt
acceptable_range1.024-1.026 sg
measuring_toolrefractometer, calibrated with reference fluid
mix_timecirculate several hours to overnight before use
Red flags — act now
- Salinity outside 1.024-1.026 sg
- Relying on a cheap swing-arm hydrometer as your only measurement
Sources
- Reef Tank Cycling Research — Fishless Cycling protocol (correct salinity before cycling)
- Reef Tank Cycling Research — Post-Cycle Stocking Strategy (salinity range)
- Reef Knowledge Base — Water Chemistry & Dosing targets
- Reef Knowledge Base — Water Preparation (RO/DI, Salt Mix)
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