The hammer coral had been sulking for a fortnight. Not dying just closed up tighter than it should be, tissue pulled back, no real reason for it. Alkalinity was 8.1. Calcium 425. Magnesium fine. Nitrate and phosphate both sitting exactly where they'd been for months. Every number on the sheet said the tank was healthy, and the tank plainly wasn't.
Then someone asked what the salinity was, and the honest answer was "1.025, according to a plastic swing-arm hydrometer I've had since 2019." A borrowed refractometer put it at 1.031.
Quick answer: Reef tank salinity should sit at 1.025-1.026 specific gravity, or 34-35 ppt natural seawater is about 35 ppt. Measure it with a refractometer calibrated against a 35 ppt seawater standard, not with RO water and not with a swing-arm hydrometer. Because evaporation removes water and leaves the salt behind, salinity always drifts upward, and the fix is topping off with pure RODI water, never saltwater.
What should reef tank salinity actually be?
Natural seawater runs about 35 ppt, and that's the honest target for a mixed reef. There's a workable band around it, and as with every other parameter sitting stably at 1.025 beats bouncing between 1.024 and 1.027 on your way to 1.026.
| Reading | What it means |
|---|---|
| Below 1.023 (30 ppt) | Too low for corals and inverts; fish will tolerate it, corals won't thrive |
| 1.023-1.024 (31-33 ppt) | Fish-only systems and some hyposalinity treatments live here |
| 1.025-1.026 (34-35 ppt) | The reef range corals, inverts and fish all comfortable |
| ~1.0264 (35 ppt) | Natural seawater, and the number to steer toward if you want one |
| Above 1.027 (36+ ppt) | Usually the result of evaporation nobody topped off; corals close up and stay closed |
The catch with specific gravity is that it's a density measurement, so it moves with temperature. Every SG figure above assumes water at about 25°C. Salinity in ppt doesn't have that problem, which is why more reefers are quietly switching to talking in ppt.
Hydrometer, refractometer, or digital meter?
This is where most salinity problems actually start. The instrument, not the tank.
| Tool | Roughly costs | The honest verdict |
|---|---|---|
| Swing-arm hydrometer | £10-15 | Salt creep, trapped bubbles and a plastic arm that ages. Can read a full 0.005 SG out and give no clue it's doing so |
| Glass floating hydrometer | £15-25 | More accurate than the swing-arm, but fragile and awkward to read |
| Optical refractometer | £25-50 | The hobby standard. Accurate once calibrated properly. Get one with ATC (automatic temperature compensation) |
| Digital salinity meter | £60-150 | Measures conductivity, no eyeballing a blurry line. Needs its own calibration routine and periodic probe care |
For most reefers the refractometer is the right call. But it only earns that reputation if you calibrate it correctly, and almost nobody does at first.
Why calibrating with RO water isn't enough
Here's the trap. Your refractometer came with a little pipette and the instructions said to zero it with distilled or RO water. So you did, the crosshair sat perfectly on zero, and you assumed you were done.
You're not. Zeroing the instrument at 0 ppt tells you nothing about how accurately it reads at 35 ppt, which is the only reading you actually care about. Worse, most cheap refractometers sold for aquarium use are internally scaled for sodium chloride brine, not for real seawater and seawater contains a whole spread of other ions with slightly different optical behaviour. The refractive index of genuine 35 ppt seawater is 1.3394, and an instrument calibrated only at zero can be several points off by the time it reaches that end of the scale.
The fix costs about a tenner. Buy a bottle of 35 ppt calibration solution, drop it on the prism, and adjust the screw until it reads 1.026 / 35 ppt. Now the instrument is accurate at the point on the scale you use it. Recheck it monthly, and any time you drop it, leave it somewhere hot, or start seeing a reading you don't believe.
Why does salinity creep up on its own?
Because evaporation is a one-way filter. Water leaves your tank as vapour; salt does not. Every litre that evaporates concentrates the salt in everything left behind.
In an open-top tank with strong lighting and a running skimmer, that can easily be several litres a day. Leave a system untopped over a long weekend and you can watch salinity climb a full point of specific gravity. This is exactly why an auto top-off is one of the highest-value bits of kit in reefkeeping not because it does something clever, but because it does something boring, continuously, while you're at work.
Two rules follow from this, and they're the ones people get wrong:
Top off with fresh RODI water only. You're replacing water that left, not salt. Topping off with saltwater compounds the problem every single time.
Water changes use mixed saltwater at your tank's salinity. Match the new water to the tank before it goes in, and check it with the same refractometer you use on the display.
Why does top-off water have to be RODI?
Because it's the one thing you add to the tank almost every day, and whatever's dissolved in it accumulates.
Tap water is fine to drink and genuinely bad for a reef. It routinely carries phosphate and silicate which is to say, algae fuel and diatom fuel along with nitrate, chlorine or chloramine, and copper leached from household pipework. Copper is the one worth pausing on: it's lethal to shrimp, snails and corals at concentrations far below anything a water utility cares about.
An RODI unit deals with this in stages. Sediment and carbon filters take out particulates and the chlorine/chloramine that would otherwise destroy the membrane. The reverse osmosis membrane does the heavy lifting, typically rejecting 90-98% of total dissolved solids. Then the deionising resin polishes off what's left including several ions that specifically matter to a reef tank and that the membrane alone won't fully remove.
What TDS should your RODI water read?
Zero. Not "close to zero." A cheap inline TDS meter is the whole quality-control system for your water, and there are two numbers worth watching.
| Reading point | What you want to see |
|---|---|
| Tap water in | Whatever your area gives you this is your baseline, note it down |
| After the RO membrane | Roughly 3-10% of your tap reading. Higher means the membrane is failing |
| Final RODI output | 0 ppm. Anything above 0 means the DI resin is exhausted |
The moment that final number ticks up to 1, the resin is spent and unfiltered ions are going into your tank. Change it. DI resin is cheap; a phosphate problem you spend six months chasing is not.
How do you fix salinity that's already wrong?
Slowly. This is the part where reefers panic and cause more harm than the original problem.
If salinity is high, top off with RODI water over several days rather than dumping in a bucket. If it's low, mix your top-off replacement slightly saltier than usual, or do a small water change with saltwater mixed a touch high. Either way, aim to move it no faster than about 0.001 SG roughly 1 ppt per day. Livestock will happily tolerate a gradual drift they'd be shocked by if you did it in an hour.
And then, once it's right, do the boring thing: check it weekly, log it, and leave it alone. Salinity isn't a parameter you tune. It's one you keep still, so that every other number you test actually means what it says.