Fish Tank Buildup

Why is my water disappearing?

I have a 10 gallon tank for my betta fish. He has a heater and filter. For some reason the water level is lowering every few days. No leaks are in the tank. I can tell the water is going down by the water mark buildup. Where is the water going?

Its evaporation. I know what you mean though. This happens especially if you live in South Carolina(like me) with a lot of humidity

Ideal Home Show 2010 build up