What drives how fast an aquarium evaporates?

Evaporation is driven by the difference between the water-vapour pressure at the warm water surface and the vapour pressure of the surrounding room air. Warmer water and drier, less humid air both increase that gap and speed up evaporation. Surface area, air movement, and a lid also matter.

Why does only surface area matter, not tank volume?

Water leaves only at the air-water interface, so a wide shallow tank evaporates faster than a tall narrow one of the same volume. Sumps, overflows, and open lids add surface area too, which is why reef systems with sumps evaporate more than their display footprint suggests.

How big should my ATO reservoir be?

Size it for how long you want to leave the tank unattended. The tool multiplies the daily evaporation estimate by one, three, and seven days. A reef keeper going away for a week should pick the seven-day figure and add a margin, since hot weather raises the rate.

Should I top off with fresh or salt water?

Always top off evaporation with fresh RODI water, never saltwater. Only pure water leaves when the tank evaporates, so the salt stays behind and salinity rises. Replacing it with saltwater would push salinity steadily higher and harm livestock.

How accurate is this estimate?

It is an engineering estimate, not a guarantee — real evaporation also depends on air movement, whether the tank is lidded, and lighting heat. Use it to size a reservoir with margin, then measure your actual daily top-off over a few days and adjust.

Aquarium Evaporation & ATO Top-Off Calculator

Every reef and marine tank loses pure water to evaporation, concentrating the salt left behind and demanding a steady supply of fresh top-off water. This calculator estimates how much your tank evaporates per day from surface area, temperature, and humidity, then sizes an auto-top-off reservoir for one, three, or seven days.

How it works

Evaporation rate rises with the vapour-pressure deficit — the gap between the saturated vapour pressure at the water surface and the actual vapour pressure of the room air — and with surface area:

e_sat(T)   = saturation vapour pressure of water at temperature T
deficit    = e_sat(Twater) − RH × e_sat(Troom)
daily L    = area(m²) × coefficient × deficit
reservoir  = daily L × days

Warmer water raises e_sat(Twater), and drier air lowers the RH × e_sat(Troom) term, both widening the deficit and speeding evaporation. The coefficient is tuned so a typical open reef surface in average room conditions matches observed top-off rates.

Example and tips

A 0.18 m² reef surface (about 60 × 30 cm) at 26C water in a 22C, 50 percent humidity room loses on the order of 1 litre per day, so a seven-day trip needs roughly a 7 litre reservoir plus margin. Always top off with fresh RODI water, never saltwater, because only pure water evaporates. Add a lid to cut evaporation, and measure your real daily top-off over a few days to refine the reservoir size.