How much water does an aquarium evaporate per day?
The hobby answer is "half a gallon to a gallon a day per hundred gallons," and it's a fair summary of an enormous range. Evaporation doesn't care about your gallons at all — it cares about the water surface, how warm the water is, how dry the room air is, and how fast that air moves across the surface. A 100 gallon system with a 48 × 18 in display and an open sump at 78 °F in a 72 °F, 40 % humidity room with still air comes out at about 0.86 gal (3.3 L) a day by the estimate this page uses; put cooling fans on it and it's 2.81; put a glass lid on and it's 0.13. That spread is the hobby's range, and it's why the number is worth estimating for your setup rather than quoting.
| Same 100 gal system… | gal/day | L/day | vs still air |
|---|---|---|---|
| Still air, open top | 0.86 | 3.3 | 100% |
| Room air moving (open sump, HVAC) | 1.41 | 5.4 | 165% |
| Cooling fans on the surface | 2.81 | 10.6 | 327% |
| Glass lid, still air | 0.13 | 0.5 | 15% |
| Humid room (70 % RH) | 0.55 | 2.1 | 64% |
| Dry winter room (68 °F, 25 % RH) | 1.04 | 4 | 122% |
Computed by this page's engine for a 48 × 18 in display plus a 20 × 12 in open sump surface, water 78 °F, room 72 °F / 40 % RH unless stated. Treat any single number as ±50 %.
Water evaporates in proportion to the difference between the humidity right at the surface (saturated, at the water's temperature) and the humidity of the room air, times a coefficient that grows with air speed. That's the mass-transfer equation used for indoor swimming pools (Carrier's form, as ASHRAE prints it), with the correction ASHRAE recommends for quiet, unoccupied surfaces. The physics is solid; the inputs aren't. Nobody knows the air speed over their tank to better than "still-ish" or "fans," and humidity swings 20 points between seasons — so treat the number as a planning figure. Everything this page does with it (reservoir days, monthly RO, kalk capacity, salinity creep) is fine at that precision. Your ATO's actual consumption over a week is the truth; enter that elsewhere on the site once you have it.
How big should my ATO reservoir be?
Days you want to be able to ignore it, times daily evaporation, plus margin. For the example system a 5 gallon jug lasts about 6 days — a week, which is the sensible minimum (a weekend away is three days; a week's holiday is seven). Ten gallons is comfortable; a 20 gallon reservoir is what people with fans and open sumps end up buying. Two constraints push the other way: RO/DI water sitting for weeks in a lidded container is fine, but a reservoir big enough to overflow the tank if the ATO sticks on is a flood risk — size it, or the sump's free space, so a runaway ATO can't put more water in than the sump has room for. That's a stronger argument for a float-switch backup or a dosing-pump ATO than for a small jug.
Salt doesn't evaporate. On the example system, each day untopped raises salinity by about 0.3 ppt; a week away with a stuck float is +2.2 ppt — 35 to 37.2, from "perfect" to "past the top of the window" without anyone touching the tank. Faster tanks (fans, big surface, small volume) creep faster; a nano can move a full ppt a day. That is the whole case for automatic top-off: not convenience, but a parameter that otherwise drifts every single day. The salinity adjustment tool undoes the drift when it happens; the ATO prevents it.
Can kalkwasser in the top-off carry my alkalinity?
Only as much as evaporates. Saturated limewater delivers a fixed amount of alkalinity per liter, and the ATO can only push in what the tank lost — so evaporation is the ceiling. The example system's 0.86 gal a day of top-off, all as saturated kalk, can supply about 0.98 dKH per day of alkalinity (and matching calcium). A low-demand mixed reef using 0.3–0.6 dKH a day is covered; an SPS system eating 1.5 dKH a day isn't, and needs two-part or a reactor for the rest. The calculator's "kalk in the ATO" line does that comparison for your surface and volume; the alkalinity dosing calculator takes the evaporation figure to check a kalk dose directly.
How much RO/DI does a reef use in a month?
Evaporation, times thirty, plus water changes. The example's 0.86 gal a day is 26 gal a month for top-off alone; add 10 % weekly water changes on 100 gallons and the RO unit is making about 66 gallons a month — a useful number when sizing an RO/DI system, and when deciding whether the fans that keep the tank cool in summer are worth the extra 2 gal a day they cost.