Hi
Like previous poster, I guess your tank is empty. Break off as much of the background as possible to expose silicone adhesive. If it is too difficult to remove mechanically then I believe there are silicone disolvers/softeners available in the DIY shops.
If this works (I have never tried it) then be sure to thoroughly clean the tank after as the disolver is bound to contain nasty stuff for fish! Also, keep it well away from any silicone that has been used to construct your tank!!!!!!!