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- Only pure water evaporates. The salt stays, so every gallon that leaves pushes salinity up.
- New salinity = old salinity × V ÷ (V − water lost). A 50-gallon system at 35 ppt that loses 2 gallons is at about 36.5 ppt (SG 1.0275).
- Top off with fresh RO/DI water, never saltwater. An auto top-off (ATO) does it as the water leaves.
- The main ATO risk is the opposite problem: a stuck sensor that empties the whole reservoir. A 5-gallon reservoir dumped into a 50-gallon system drops 35 ppt to about 31.8 ppt. Use two independent shut-offs and a small reservoir.
Why salinity creeps up
Seawater is water with salts dissolved in it. When the tank evaporates, only the water leaves as vapor, and all the salt stays behind in less water. The tank's salinity rises even though you never added any salt.
That is why reefers keep the water level steady with fresh water, not saltwater. Topping off with saltwater adds more salt every time, and salinity ratchets up week after week. Saltwater only goes in to replace saltwater that left, as in a water change or skimmate dumped from the skimmer cup.
Natural seawater sits near a practical salinity of 35, which oceanographers use as the reference point for Standard Seawater (Millero et al. 2008). Most reef tanks are kept at about 33.5–35 ppt, or 1.025–1.026 specific gravity. Our salinity converter turns ppt into specific gravity and back.
How fast it climbs: the math
New salinity = starting salinity × system volume ÷ (system volume − water evaporated)
Example: 50 US gal at 35 ppt loses 2 gal → 35 × 50 ÷ 48 = 36.5 ppt.
Use the total water in the system, display plus sump, not the tank's label size. The reef tank volume calculator will get you close. Here is what happens with no top-off, starting at 35 ppt (SG 1.0264):
| System water | Lose 1 gal | Lose 2 gal | Lose 3 gal | Lose 5 gal |
|---|---|---|---|---|
| 20 US gal | 36.8 (1.0278) | 38.9 (1.0293) | n/a | n/a |
| 50 US gal | 35.7 (1.0269) | 36.5 (1.0275) | 37.2 (1.0281) | 38.9 (1.0293) |
| 100 US gal | 35.4 (1.0266) | 35.7 (1.0269) | 36.1 (1.0272) | 36.8 (1.0278) |
Values are ppt with specific gravity at 25 °C in brackets, calculated with the same seawater equation our salinity converter uses. Small systems swing fastest: a 20-gallon nano losing 2 gallons over a long weekend goes from 35 to nearly 39 ppt.
Why corals care about the swing
Corals do not have a way to keep the salt balance of their tissues independent of the water around them, so a fast change in salinity is a direct stress. Lab work on Stylophora pistillata, a common reef-tank stony coral, found that exposure to reduced-salinity seawater cut the photosynthetic efficiency of the coral's symbiotic algae and led to bleaching, in the light or the dark (Kerswell & Jones 2003). That study looked at low salinity, the direction a failed ATO pushes a tank. High salinity from missed top-off is a stress too, and the steady fix is the same: keep salinity where your corals are used to it.
If you find salinity well off target, bring it back gradually over a few days rather than all at once. Raise it by topping off with saltwater instead of fresh for a few days, or lower it by swapping a little tank water for RO/DI. Retest each day with a calibrated instrument (see how to calibrate a refractometer).
How to measure your evaporation
- From the ATO reservoir: mark the level, wait 24 hours, and measure how much it dropped. That volume is your daily evaporation.
- Without an ATO: mark the sump waterline after topping off, then measure how much RO/DI it takes to return to the mark a day later.
- Turn it into a percent: daily evaporation ÷ system volume. Losing 0.75 gal from 75 gal is 1% a day, which on its own would raise 35 ppt by about 0.35 ppt per day.
Evaporation depends on surface area, water temperature, room humidity, fans, and lids, so it changes with the seasons. Heating season usually dries the air and increases it. Remeasure whenever you add a fan, a lid, or a chiller, and expect a cooling fan over the sump to raise it (that is how fans cool a tank; see evaporative cooling).
How an auto top-off works
An ATO is a small pump in a reservoir of RO/DI water, switched by a sensor at the sump's normal waterline. When the level drops a little, the pump runs until the sensor sees the water back at the line. The tank never swings more than the sensor's small dead band.
- Float switches are simple and cheap. They can stick from snails, coralline algae, or salt creep, so clean them every few months.
- Optical sensors have no moving parts but can be fooled by bubbles, splashing, or a film on the lens. Keep them out of the bubble path and wipe them when you clean the sump.
- Sensor placement: mount it in a calm section of the sump where the level only changes from evaporation. A sensor in the return section also sees the water that drains back when the return pump stops, which is fine as long as the sump can hold that drain-back.
Failsafes that keep an ATO from crashing a tank
An ATO that fails "off" is a slow problem: salinity climbs over days. An ATO that fails "on" is fast: the pump keeps running until the reservoir is empty or the sump overflows. Plan for both.
| Fresh water dumped | Into 50 gal at 35 ppt | Into 100 gal at 35 ppt |
|---|---|---|
| 5 gal | 31.8 ppt (1.0240) | 33.3 ppt (1.0251) |
| 10 gal | 29.2 ppt (1.0220) | about 31.8 ppt (1.0240) |
- Two independent shut-offs. Pair the main sensor with a backup high-level float, or use a unit or controller that stops the pump after a maximum run time.
- A reservoir sized to the risk. A few days of evaporation is plenty. The table above shows what a full dump would do to your system, so pick a size you could survive.
- No siphon. Keep the top-off line's outlet above the sump waterline, or drill a small anti-siphon hole, so water cannot keep flowing after the pump stops.
- Low-reservoir protection. Some units cut the pump when the reservoir runs dry so it does not burn out, and some can alert your phone.
- Check salinity weekly anyway. An ATO keeps salinity steady only if the starting point was right and nothing else is removing or adding salt.
Topping off with kalkwasser
Because top-off water goes in every day, many reefers dissolve kalkwasser in it to feed calcium and alkalinity at the same time. That works well, but it raises the stakes of a stuck ATO, since too much kalk at once spikes pH. Use a slow pump and the same failsafes, and keep the daily dose in line with your alkalinity use. Our kalkwasser dosing guide covers the details, and the kalkwasser calculator shows whether your evaporation can carry your tank's demand.
ATO gear we would start with
See our auto top-off buyer's guide for how these compare.
- Two sensor types in one unit: Tunze Osmolator 3, an optical sensor backed by a thermal safety sensor, for sump systems. It has no siphon break, so set the outlet above the waterline.
- Compact, for nanos and AIOs: XP Aqua Duetto2, with a small dual optical sensor that fits tight rear chambers.
- App-connected: Red Sea ReefATO+, which ties into the Red Sea app alongside their other gear.
- To check the result: Seawater-specific refractometer with ATC, calibrated with 35 ppt fluid.
Common top-off mistakes
- Topping off with saltwater.
- Using tap water for top-off, which adds nitrate, phosphate, and silicate every day (see RO/DI water for reef tanks).
- A huge reservoir with a single float switch and no backup.
- Calculating salinity changes from the tank's label size instead of the real system volume.
- Correcting a big salinity error in one day.
FAQ
Should I top off a reef tank with saltwater or fresh water? Fresh RO/DI water. Only pure water evaporates, so the salt stays behind in the tank. Topping off with saltwater adds salt every time and pushes salinity steadily higher. Saltwater goes in only during water changes.
How much does evaporation raise salinity? New salinity = old salinity × system volume ÷ (system volume − water lost). A 50-gallon system at 35 ppt that loses 2 gallons without top-off reaches about 36.5 ppt (specific gravity about 1.0275 instead of 1.0264). The smaller the system, the faster it climbs.
How much water does a reef tank evaporate per day? It depends on the water surface area, temperature, room humidity, fans, and whether the tank has a lid, so there is no single number. Measure your own: note how much water your ATO reservoir loses over 24 hours, or mark the sump waterline and measure how much RO/DI it takes to bring it back. Recheck when the seasons change.
What is the safest auto top-off setup? Use two independent shut-offs: the main level sensor plus a backup high-level switch or a controller that stops the pump after a maximum run time. Keep the reservoir small enough that emptying it all at once would not drop salinity dangerously, and keep the pump outlet above the waterline so it cannot siphon.
Can I put kalkwasser in my ATO? Yes, many reefers top off with saturated kalkwasser, but only with a slow pump and strong failsafes, because a stuck ATO can push enough kalk in to spike pH. Make kalk only in fresh RO/DI water, and size the dose to your alkalinity use with the kalkwasser calculator.
References
- Kerswell, A.P. & Jones, R.J. (2003). Effects of hypo-osmosis on the coral Stylophora pistillata: nature and cause of 'low-salinity bleaching'. Marine Ecology Progress Series 253: 145–154. doi:10.3354/meps253145
- Millero, F.J., Feistel, R., Wright, D.G. & McDougall, T.J. (2008). The composition of Standard Seawater and the definition of the Reference-Composition Salinity Scale. Deep-Sea Research Part I 55: 50–72. doi:10.1016/j.dsr.2007.10.001
- Fofonoff, N.P. & Millard, R.C. (1983). Algorithms for computation of fundamental properties of seawater. UNESCO Technical Papers in Marine Science 44 (seawater equation used for the specific gravity values above).