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Why salinity matters
Salinity is the total amount of dissolved salt in seawater. Corals, fish and invertebrates from coral reefs evolved in water with a remarkably stable salinity of about 35 parts per thousand (ppt). Their cells balance water and ions against that level. Large or rapid swings force them to spend energy on osmoregulation and can kill sensitive invertebrates outright. Salinity also sets the baseline for every other parameter: a properly mixed reef salt at the right salinity delivers roughly the right calcium, magnesium, alkalinity and trace elements.
Units: ppt, specific gravity and conductivity
Reefers measure salinity three ways:
- Salinity (ppt or PSU). The amount of salt itself. Natural seawater is about 35 ppt.
- Specific gravity (SG). The density of the water relative to pure water. 35 ppt corresponds to an SG of about 1.026 at 25 °C (77 °F). Because SG depends on temperature, the same water reads differently when warmer or colder, which is why tools specify a reference temperature.
- Conductivity (mS/cm). How well the water conducts electricity, which rises with dissolved ions. 35 ppt is about 53 mS/cm at 25 °C. Aquarium controllers often use conductivity probes.
Most reefers target 1.025–1.026 SG (about 33.5–35 ppt). Natural seawater at 35 ppt is the reference point, and many aquarists simply run 35 ppt. Fish-only tanks are sometimes run slightly lower, but a reef should stay near natural levels.
Measuring tools
Refractometer
A refractometer measures how much the water bends light, which tracks salinity. Optical refractometers are inexpensive and accurate when calibrated. Buy one made for seawater: many cheap models are calibrated for brine or sugar solutions and read seawater slightly wrong. Most have automatic temperature compensation (ATC), but the sample and instrument should still be at room temperature.
Calibrate with a 35 ppt seawater standard, not RO/DI water. Calibrating to zero with pure water is standard practice for the zero point, but refractometers can be nonlinear, so a reading at 35 ppt can still be off. A 35 ppt calibration fluid checks accuracy exactly where you use it. Recheck every few months and whenever a reading seems odd.
Digital meters and conductivity probes
Digital refractometers and handheld conductivity meters (such as the Hanna marine salinity checker) remove eyeball error. Controller probes give continuous readings and can alarm on drift, but they also drift and need periodic calibration.
Swing-arm hydrometers
Cheap plastic hydrometers are notoriously inaccurate, especially as they age or collect bubbles and salt deposits. They are fine for a rough check but should not be trusted for a reef.
Evaporation and top-off
Only fresh water evaporates. Salt stays behind, so salinity rises as the water level falls. A medium reef can lose a gallon or more per day in a dry home. Top off evaporation with fresh RO/DI water, never saltwater. Topping off with saltwater steadily raises salinity.
An auto top-off (ATO) replaces evaporated water as it happens and is one of the best stability upgrades you can buy. Use a reservoir no bigger than a few days' worth of evaporation, or a unit with a failsafe, so a stuck sensor can't flood the tank with fresh water and crash salinity.
Salt creep, the white crust on lids and equipment, removes a small amount of salt over time. So does skimmate. That's another reason to measure salinity regularly rather than assume.
Mixing saltwater correctly
- Start with RO/DI water at about room temperature in a clean, dedicated container.
- Add the salt mix gradually while a pump circulates the water. Manufacturer instructions usually call for roughly half a cup per gallon, but always measure salinity rather than rely on volume.
- Heat to tank temperature and mix for at least several hours, often overnight, until the salt is fully dissolved and the water is clear.
- Test salinity and adjust with more salt or more RO/DI until it matches the tank.
- For a mature reef, occasionally test the new saltwater's alkalinity, calcium and magnesium too. Brands differ, and some run alkalinity high, which matters if your tank runs lower.
Avoid storing mixed saltwater for months; most brands recommend using it within a week or two, or storing it covered with circulation.
Why RO/DI water
Tap water can contain nitrate, phosphate, silicate, copper, chlorine or chloramine, and other contaminants. Even if it's safe to drink, these fuel algae, can be toxic to invertebrates, and vary from season to season. RO/DI (reverse osmosis plus deionization) removes essentially all of it and gives you consistent pure water. That's why RO/DI is considered standard for reef tanks, both for mixing salt and for top-off.
How an RO/DI unit works
- Sediment filter. Catches particles that would clog the membrane, typically 1–5 micron.
- Carbon block. Removes chlorine and organic compounds. Chlorine damages thin-film membranes, so this stage protects the membrane. Water supplies treated with chloramine need a catalytic carbon block or an extra carbon stage, since chloramine passes through ordinary carbon more easily.
- RO membrane. A semipermeable membrane rejects the large majority of dissolved solids, often in the 95–99% range when new. RO produces "waste" water that carries the rejected solids away. Typical ratios range from about 1:1 to 1:4 product to waste, depending on the unit and water pressure.
- DI resin. Mixed-bed deionization resin exchanges the remaining ions, polishing water to near 0 ppm TDS.
Monitoring TDS and when to change filters
A dual inline TDS meter, measuring after the membrane and after the DI stage, tells you exactly when maintenance is due:
- After DI, TDS should be 0 ppm. Replace the DI resin when it reads 1 ppm or more. Color-changing resin helps you see exhaustion.
- After the membrane, compare to tap TDS. When membrane rejection falls significantly (TDS after the membrane climbs relative to tap), the membrane is wearing out. Membranes typically last a few years.
- Sediment and carbon are usually replaced every 6 months or so, sooner with dirty or heavily chlorinated water. A pressure drop or slow production often means a clogged sediment filter.
RO membranes work best around 60 psi or more. Low household pressure lowers output and increases waste. A booster pump fixes that. Always let the first minute or so of production go to drain after the unit has been sitting idle, because TDS creep makes the first water out slightly worse.
A note on hyposalinity
Very low salinity, around 1.009 SG, is sometimes used to treat marine ich in fish-only quarantine. It would kill corals and most invertebrates and must never be done in a reef display. See our pests and diseases guide.
Correcting salinity safely
If salinity is off, correct it gradually, by roughly 0.001 SG (around 1–1.5 ppt) per day. Lower salinity by removing some tank water and replacing it with RO/DI. Raise it by replacing some tank water with saltwater mixed slightly higher, or by topping off with saltwater for a few days. Never add dry salt directly to a tank with livestock.
Summary
Run your reef at about 35 ppt (1.025–1.026 SG), measure with a calibrated seawater refractometer or conductivity meter, top off evaporation with fresh RO/DI using an ATO, and keep your RO/DI system at 0 TDS by changing stages on schedule.
Frequently asked questions
What salinity should a reef tank be?
Most reefs run at 1.025–1.026 specific gravity, roughly 33.5–35 ppt, with 35 ppt matching natural seawater.
Is 1.026 the same as 35 ppt?
Very close. Seawater at 35 ppt has a specific gravity of about 1.026 at 25 °C (77 °F).
How do I calibrate a refractometer for saltwater?
Zero it with RO/DI water, then confirm with a 35 ppt seawater calibration standard and adjust if needed, with both sample and refractometer at room temperature.
When should I change my RO/DI filters?
Replace DI resin when output reads 1 ppm TDS or more, sediment and carbon about every six months, and the membrane when its rejection drops noticeably.
Do I top off with saltwater or fresh water?
Fresh RO/DI water. Only water evaporates, so the salt stays in the tank.
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More reef science guides
- Alkalinity, calcium & magnesium
- Nitrogen cycle & cycling
- Nitrate & phosphate
- Lighting & PAR
- Coral types & care
- Pests & diseases
- Reef conservation & science
- Coral symbiosis & Symbiodiniaceae
- Coral bleaching science
- Ultra-low nutrient research
- Ocean acidification & calcification
- Coral restoration & microfragmentation
- ICP testing & trace elements
Ranges are common hobby targets compiled from reef chemistry literature (including Randy Holmes-Farley's articles), hobby consensus and manufacturer documentation. They're not rules. Last reviewed 2026-10-15.