What about removing it completely and replacing it either with a new, non-siliconed in cover glass or a plastic condensation cover?
I was adament to start cutting the seals and removing the cover and ledges incase it was structural.
I know many tanks dont come with these, but as it was fitted originally I thought maybe it was factored into the general strength of the tank and didnt want to rely on the 4 edge seals alone to hold the weight.
I am really struggling to find new tanks to buy on the net. Im from England and most are American sites that sell them.