Estimate daily and weekly freshwater top-off volume.
Open live calculator →A 200 litre open aquarium losing 0.6% daily needs about 1.2 litres of top-off water per day.
Base reservoir size on measured seasonal evaporation.
Convert an observed daily evaporation percentage into a practical top-off estimate for reservoir planning.
The live tool updates immediately and shows the result in the units commonly used by aquarium keepers around the world.
Measure actual loss over several normal days.
Evaporation changes with season, airflow and lids.
Marine tanks should be topped off with fresh water, not salt water.
Open-top tanks in dry or warm rooms commonly lose 0.5–1% of volume per day; covered tanks lose far less. Measure your own tank over several typical days for a reliable figure.
Salt does not evaporate—only water leaves. Topping off with saltwater raises salinity steadily. Replace evaporation with pure RO/DI fresh water to keep salinity stable.
Size it for at least several days of your measured evaporation—the weekly estimate the calculator gives is a practical minimum for a holiday-proof reservoir.