ive used plexi inside glass tanks before, but it never stuck very well. i used both caulk and silicone. the caulk just comes off it very easy. at least what i used did. i found that siliconing glass inside a glass tank worked great or obviously acrylic welded into an acrylic tank. it seems if you keep the materials the same, they work awesome, if you mix them, they never bond very strong and can easily blow out with water pressure. i'm sure its been done pleanty of times with no problems at all, its just my personal findings that its best to keep the materials the same.