I have been reading lots of posts regarding condensation building up on the lid of glass and acrylic tanks. I to have that same problem. I have a whisper 3I filter with a very cheap lid. In order to prevent water from spattering everywhere I brought the polyester filter fiber from petland discounts and put a small piece behind my filter cartridge and so far it has cut the condensation down 97%. The added benefit is that it now it helps insure more water is being filtered through the filter cartridge and insures my water is being cleaned better for my betta Junior.
I hope this helps anyone who is having a similar problem.