A lid on the tank often helps keep the evaporation level down. In my 2.5 gallon, I never lose any water because of the lid (it condenses on the lid, and then drips back down into the tank). But i prefer the open top, myself. I probably lose at least a gallon a day (!!) right now in my 55--have powerful lights and a fan on the surface trying to cool the water all the time...
Downside of a lid is that it reduces oxygen levels in the tank (less surface area interacting with fresh oxygenating air) and heats the tank up. But if you live in the very north this may be a good thing!
Regular top-offs with treated, filtered water are the best bet, really.
Oh, and do yourself a favor. Before your gouramis start to die from ammonia poisoning (ammonia is produced in fish waste, and is toxic to fish), get some "Cycle Biological Aquarium Supplement" (concentrated nitrifying bacteria; produced by Hagen) for your tank at the Local Fish Store (LFS
), an ammonia-reducing pack for your filter, and an ammonia test pack (tells you how many parts per million of ammonia are in your tank). You're may have a long road ahead to get rid of the ammonia spike. If you don't know about ammonia spikes, I suggest you read up on them on the net. They are the #1 killer of new fish, and always occur within the first 3 weeks of starting up a tank. A tank should always be cycled (with feeder fish, or zebra danios, usually) before you add fairly sensitive fish (like gouramis). If you want, I can post another message explaining the nitrogen cycle--let me know. But, do try to look it up on the web if you don't know what I'm talking about.
And don't buy any more fish for your tank until the spike is gone--you may be sufficiently under maximum biological capacity in your tank to avoid fish loss.
Sorry if you knew all of this, in which case I'm just boring you!