Evaporation and refilling (ATO)
Aquarium water evaporates continuously. Topping up must be done with fresh water (RO) because the salt does not evaporate. ATO systems automate this.
Detailed explanation
Evaporation is a daily occurrence in every open aquarium. A 300 litres reef can lose 2-5 litres per day depending on temperature, ambient humidity and ventilation.
The problem: the water evaporates but the salts remain. If you don't top up the salinity will rise. If you top up with salt water, add salt to the already concentrated salt. Topping up must always be done with RO fresh water.
An ATO (Auto Top Off) system automates refilling. A level sensor in the aquarium or sump detects when the level drops and activates a pump that draws RO water from a reserve container. The most reliable systems have a double safety sensor to avoid uncontrolled top-ups (which would dilute the salinity too much).
In ponds evaporation is enormous in summer. A 5000 litres pond can lose 20-50 litres per day in the heat. Topping up is done with a water hose but be careful of chlorine: an in-line carbon filter is better.
For freshwater aquariums, evaporation changes the volume but not the concentration of minerals critically. However, in small (nano) aquariums, even topping up with mains water alters the GH and KH over time.
Coralia Lab monitors this parameter with AI diagnostics. Tank Score alerts you when a value moves outside the recommended range for your aquarium.
Try Evaporation calculator for free