How evaporation is calculated
Water evaporates because the air right above warm water holds more moisture than the room air. The Engineering ToolBox gives a standard empirical formula: evaporation = (25 + 19 × air speed) × area × (moisture of saturated air at water temperature − moisture of room air).
We compute moisture with the Magnus vapour pressure formula at sea-level pressure, assume gentle indoor air movement, and then reduce the result for covers: a hood with gaps lets through about half, a tight glass lid about 15% (our estimate, since lids vary widely).
Typical evaporation rates
| Water surface | Room humidity | Open top | Per week (open) | Glass lid |
|---|---|---|---|---|
| 60 × 30 cm | 30% | 1.82 L | 12.7 L | 0.27 L |
| 60 × 30 cm | 50% | 1.42 L | 10.0 L | 0.21 L |
| 60 × 30 cm | 70% | 1.02 L | 7.1 L | 0.15 L |
| 90 × 45 cm | 50% | 3.20 L | 22.4 L | 0.48 L |
| 120 × 45 cm | 30% | 5.46 L | 38.2 L | 0.82 L |
| 120 × 45 cm | 50% | 4.27 L | 29.9 L | 0.64 L |
| 120 × 45 cm | 70% | 3.06 L | 21.4 L | 0.46 L |
Why evaporation matters in saltwater tanks
Only water evaporates; the salt stays behind. A 90 × 45 cm reef holding 200 L at 26 °C loses about 3.6 L a day. Without top-off, salinity climbs from 35 ppt to about 35.6 ppt in a day and 40 ppt (SG 1.030) in a week, which is dangerous for corals and invertebrates.
Always top off with fresh RO/DI water, never saltwater. An automatic top-off (ATO) with a reservoir holding at least a week of evaporation keeps salinity steady.
How to reduce evaporation
- Fit a lid. A tight glass lid is the biggest single change, though it also reduces gas exchange, so keep the surface moving.
- Lower the water temperature to the low end of your fish’s range if it suits them.
- Keep fans off the surface unless you need them for cooling; evaporation is how fans cool a tank.
- Check room humidity. Heating dries indoor air in winter, so evaporation usually rises then.
