need a tight fitting lid. they are escape artists. If you don't have a tight fitting glass lid, i.e like a wood canopy, then get some of that... (I forget the term) stuff they use to put into 4 X4 foot florecent ceiling fixtures, it's got a bunch of little squares in it, some are chromed and some are plain. use the plain stuff. (available at Home depot in the lighting section) and cut it to fit inside the lip of the tank.
maybe some one else can think of what they call this stuff.
Sorry for you loss