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Salinity Drift and Top-Off Water in a Reef Tank
Salinity creeps up because only fresh water evaporates — the salt stays behind. Replace the lost volume with RO/DI top-off water only, never salt mix. An auto top-off (ATO) does this in small, constant amounts, which keeps salinity near 35 ppt (≈1.025–1.026 SG). Use a reliable sensor, a second independent fail-safe, and a reservoir sized for several days of evaporation.
A reef tank loses water to the air every day. Skimmers, open lids, fans and warm lighting all speed that up. The water that leaves is pure H2O. The salt does not leave with it. So unless you replace what evaporated, every litre left in the tank gets saltier. Slow drift like this rarely kills a coral overnight. It does wear on a system, and it is easy to prevent.
Why salinity creeps up
Imagine a 400-litre system that loses 4 litres a day. Over a week that is 28 litres of fresh water gone, around 7% of the volume. With no top-off, salinity would rise by roughly the same fraction, from 35 ppt to well over 37 ppt. Most keepers never let it get that far, but even a half-point drift between water changes adds stress that you can avoid.
For how to read and calibrate the numbers themselves, see our guide to salinity and specific gravity. This guide covers the other half: keeping the number steady.
Evaporation versus salt creep
Two different things get called "drift", and the fixes differ.
| Problem | What you see | Usual fix |
|---|---|---|
| Evaporation | Water level drops in the sump; salinity climbs steadily | Top off with RO/DI water |
| Salt creep | White crust on the rim, pump and lid; no change in salinity | Wipe it off and rinse equipment; it is residue, not a salinity fault |
| Over-topping | Salinity falls; level too high | Check ATO sensor position and pump run time |
| Wrong top-off liquid | Salinity jumps or drops after a refill | Top off with fresh RO/DI water; use salt mix only at water changes |
Salt creep is cosmetic. Dried spray leaves crystals around the tank. It does not mean your salinity is wrong. Evaporation is the real driver of drift.
Top-off water: RO/DI only
Always top off with reverse-osmosis, de-ionised (RO/DI) water. Tap water carries phosphate, silicate, copper and other dissolved solids. Every litre you add on top of evaporation leaves those solids behind, because they too stay when water evaporates. Over months this feeds algae and builds up unwanted elements. Check your RO/DI output with a TDS meter and change filters when the reading rises above zero.
Choosing an ATO sensor
An ATO watches the water level in your sump and runs a small pump when it dips. The sensor is the part that matters most.
| Sensor type | Strengths | Watch out for |
|---|---|---|
| Mechanical float | Cheap and simple | Can stick from salt or debris; clean it regularly |
| Optical | No moving parts; reacts to the water surface | Algae or film on the lens can fool it; wipe it often |
| Dual or redundant sensors | One fails, the other still protects the tank | Costs more; needs two mounting points |
Mount the sensor where the level changes only because of evaporation. Do not place it where return flow, skimmer foam or wave action moves the surface. Rapid on-off cycling is a sign of a poorly placed sensor.
Fail-safes and the reservoir
An ATO that fails open can pour a whole reservoir of fresh water into your sump. That drops salinity fast and can overflow it. Build in protection.
Size the reservoir for a few days of evaporation, not weeks. Measure your daily loss first. A reservoir that holds roughly three to seven days of top-off is easy to refill and easy to keep clean. Keep it covered, and clean it every so often to stop slime and algae growing in the water. Place the pump so it draws from the bottom but not from any sediment.
No ATO yet? Top off by hand
You do not need hardware to hold salinity steady. A manual top-off works if you do it often. Mark the normal water line in the sump with a strip of tape. Every day, add RO/DI water slowly until the level returns to the mark. Pour it in a thin stream, ideally near a return or in the sump's calm chamber, so the new water mixes before it reaches the display. Daily top-offs of a small volume are far kinder than a big weekly refill, because the swing in salinity stays small.
If you go away for a weekend, an ATO pays for itself. A manual routine cannot match the steady hand of a sensor, and that is the main reason most established reefs end up with one.
Tracking drift in ReefDeck
An ATO hides evaporation. That is its job, but it also hides a problem. If the daily top-off volume suddenly doubles, something has changed: a leak, a hotter room, a faulty sensor. Note the reservoir level each time you refill it, and take a salinity reading at least weekly with a calibrated refractometer (see how to calibrate one). In ReefDeck you can log salinity with every test and keep notes on top-off volume, so a slow upward trend or a change in usage shows up on the chart instead of in your corals. ReefDeck records and displays your own data. It does not diagnose or prescribe.
Log salinity and top-off volume in ReefDeck
ReefDeck is a free, offline reef logbook. Record salinity next to every other test, note how much top-off water you add, and see drift on a trend line before it becomes a problem. No account needed, and your data stays on your device.
Open ReefDeck — it's free → Works on phone and desktop · installs as an app · exports to CSV anytimeFrequently asked questions
Why does reef tank salinity keep rising?
Only fresh water evaporates. The salt stays in the tank, so the same amount of salt sits in less water and salinity climbs. Replacing the lost volume with RO/DI water brings it back to target.
Should I top off with saltwater or fresh water?
Top off with fresh RO/DI water. Evaporation removes water only, so you replace water only. Use salt mix when you do water changes, not to replace evaporation.
How big should an ATO reservoir be?
Measure your daily evaporation, then choose a reservoir that holds around three to seven days of top-off. That is large enough to avoid daily refills and small enough that a failure cannot drastically change your salinity.
Is salt creep a sign my salinity is wrong?
No. Salt creep is dried splash and spray that leaves crystals on the rim, lid and equipment. It is cosmetic. Check actual salinity with a calibrated refractometer.