generally glass tanks are only for short time quarantines, or momma rats with babies. and even then some people use smaller wire cages.
glass tanks don't have the wonderful circulation that wire cages do, and we all know (even though we love them) that rats can get pretty stinky pretty fast. also, most rats are genetically predisposed to having respiratory problems, so good air circulation is very important to them. technically, if you clean the cage very thoroughly every day and take them out plenty, there's nothing wrong with a glass tank. it's just a lot easier with a wire cage, and plus, you get all that space your ratty can climb for fun, and places to hook up hammocks and tubes and stuff to play in.
how big is your tank? you might be able to get a wire topper for it. check out martin's cages for aquarium toppers online. it's just a wire cage that sits on top of an aquarium to provide extra space and air.