My 20 loses about 1-2 inches a week due to the hob cutout being a bit too large, did a better job the on the 10g shrimp tank and it doesn't seem to lose any water. All it is is evaporation. 
FSM;2686593; said:its possible that flying water ninjas enter you house at night and drink out of your tank with straws.
probably just evaporation though.

