Does a sump lose water through condensation?

[tcarswell]

The tank is covered but not the sump

and I guess evaporation is what I ment rather then condensation

Id cover the sump to discourage evaporation. A tank with a sump is going to have almost all of the water loss from evaporation show in the sump so its best to keep it covered and to top it off using RO water. That way you dont get a huge buildup of the left over minerals etc