If it stuck on then I would suggest using a hair dryer to soften the glue and peel it off carefully. If the tank is glass then you can use a razor blade to help... CAREFULLY!
If the tank is glass then you can get a product called 'Tar and Glue Remover' and other similar products that are made for removing glue residue and tar from car paintwork. I used to work as a vinyl designer/fitter making signs etc and we used it all the time to remove glue, it takes it off in seconds.
A word of caution however, use it sparingly as I don't imagine it will do the silicone seal much good, I would avoid getting it on that at all. Also, I woudlnt use this on an acrylic tank as I don't know how it will affect it. I have used such products on plastic before with no adverse effects, still I wouldn't feel comfortable given the pressures etc involved.
Hope this helps
Aaron