Predict daily water evaporation and calculate top-off requirements based on tank surface area, temperature, and humidity.
Every open or partially covered aquarium loses water continuously to evaporation, and that loss concentrates salts, nutrients, and trace elements while gradually lowering the water line below the filter intake. This calculator estimates daily, weekly, and monthly evaporation from your tank surface area, room temperature, humidity, and whether the tank has a glass lid, an open top, or an in-between configuration. The result lets you size an auto top-off (ATO) reservoir, plan freshwater make-up frequency, and budget RO/DI consumption for reef and dwarf-shrimp systems.
Evaporation scales with surface area, the temperature difference between water and room air, and the air's ability to hold more moisture (a function of room temperature and relative humidity). The model treats an open-top tank as the baseline, applies a 0.5 reduction factor for a glass lid, and uses room humidity to scale the drying potential. Output is liters per day, the recommended weekly top-off volume, and a suggested ATO reservoir capacity so you do not run the reservoir dry between visits.
Enter your tank's surface area (length × width) and room conditions. Having a lid significantly reduces evaporation.
Evaporation rates can increase dramatically in dry winter conditions or when using cooling fans in summer.
Use the weekly and monthly estimates to size your Auto Top-Off (ATO) reservoir appropriately.
Open-top tanks typically lose 1–3% of volume per day. A 200 L tank may lose 2–6 L per day under typical room conditions. Lidded tanks lose 0.5–1% per day. Marine tanks with sumps and heaters often see the highest evaporation.
Add a glass or acrylic lid (cuts evaporation 50–70%). Lower the heater set temperature by 1–2°C in summer. Reduce light intensity if it heats the water. Higher room humidity also slows evaporation.
Yes. Only fresh water evaporates, so salt concentration rises as water level drops. A reef tank can shift from 35 ppt to 36–37 ppt within a day of skipped top-off. Always top off with fresh RO/DI water (not saltwater) and use an ATO if possible.
Multiply daily evaporation by the number of days you want autonomy. For a 200 L tank evaporating 4 L/day, a 30 L reservoir gives about a week of headroom — enough for short trips. Always include a safety low-water cutoff.
Indoor heating dries room air to 20–30% relative humidity, which dramatically increases the evaporation gradient at the water surface. Heaters also run more in winter, slightly raising water temperature. Both effects can double winter evaporation.