Epoxy resin can be used to seal the inside of a plywood tank, or to seal decorations or something to make them aquarium safe....do you mean like to seal a tank? I don't know how well it bonds to glass but I would use silicone, its much cheaper and you know it will work.
Id have to believe no it wont because it doesn't flex like silicone with tank water pressure much to hard. Would prolly just crack or seperate but Im no expert on dynamics
Yes if you buy a liquid two part epoxy, you can paint it on things and it will seal whatever you are coating from water. It may need a few coats. I'm talking liquid epoxy resin, like comes in a paint can, not 2-part epoxy adhesive.