you can't glue plastic to glass, at least, not using silcone. You can sorta stick a piece of plastic in using silcone around it, but it won't hold up to any pressure or abuse.
ideally you want to use glass ... silcone (being made of the same material glass is) works very good at gluing glass to itself...
the trick is having a very smooth, clean and tight fit so just a very thin amount of silcone is between your new glass and tank wall. acheive this by getting your glass cut to the same width as the sides of the tank (as exact as possible), and then have a helper spread your tank walls apart slightly, fit the new glass in making sure it is square and then let the walls go that should squeeze in on the new piece perfectly, ensuring a good strong and permant bond.
remember to let it cure 48 hrs before water testing
another idea for plastic and retaining some strenght would be a marine epoxy that claims to bond both glass and plastic.