Aquarium Glass Cover Has Come Unstuck

Hi, welcome to the forum 

Can you tell us how the glass used to be attached to the tank?

For instance, with my tank the two pieces of glass slide along two plastic strips (at the front and back of the tank) which are supported by strips of glass which are siliconed to the tank walls. Was your tank like this or were the runners for the glass covers attached directly to the tank walls?

I think there is a silicone that can be used under water with fish in the tank.

Aha! Found it  https://www.dohse-aquaristik.com/en/p/11965/Fix-Underwater-Adhesive

There are a few on-line shops selling it in various sized packs.