that would be due to condensation
have you got a condensation tray?
will help remedy the problem slightly.
I have the same problem with my planted tank and turtle tank.
Yeah condensation on the lid, runs down the back of the tank, it's a new little one I've set up in the kitchen for galaxy rasbos, but it's a pentagon shape and you can't get trays like that shape lol
My local Pets@home sells condensation trays, I just bought a rectangle one and cut it to size to fit my hexagonal tank, works a treat and really cheap as well only a couple of quid.