The short answer
The single most effective way to cut evaporation is a lid or cover glass over the tank. Beyond that, a slightly lower water temperature and less surface agitation both slow how fast water leaves. Evaporation is normal and not harmful, but reducing it means fewer top-ups, slower-rising hardness and less limescale on the glass.
What drives evaporation
Water leaves fastest when the surface is warm, exposed and disturbed. Heat gives water molecules the energy to escape, an open top lets them go, and agitation from filter outflows or air stones increases the surface area in contact with the air. Tackle those three and the level holds far longer.
The practical fixes
- Fit a lid or cover glass. This is by far the biggest lever — a cover traps the humid air above the water and dramatically slows loss. It also stops fish jumping and helps hold heat.
- Ease off the temperature. If your tank runs warmer than it needs to, a slightly lower (still species-appropriate) temperature reduces evaporation and eases the heater’s workload.
- Reduce surface agitation. Strong outflows, spray bars aimed at the surface and vigorous air stones all speed evaporation. Gentle circulation still oxygenates without churning water off as fast.
Next steps
A lid pays off in several ways at once — see do I need a lid on my aquarium?. Because evaporation concentrates minerals and leaves scale, see removing limescale from the glass and what is TDS?. Top up with treated water using a good conditioner, and browse maintenance gear for lids and tools.