Unless you have a suicidal fish, it's best to go topless on the tank, better gas exchange, less CO2
in your tank. As far as evaporation goes, on a 10g without the lid you'll have to top off about everyday. Any fluctuation on such a small tank can be detrimental to your inhabitants. Just keep a spare gallon jug with RO
water in it to top off daily. Do not, and I stress out, do not use tap water. Even if "treated" it'll still have all the nutrients algae needs to flourish. It's best to start off right than to try and fix later.