Epoxy may leach chemicals into the water, aquarium silicone is the safest thing to use.
If you can find GE silicone I, it is the same thing that is in the tubes of All-Glass aquarium sealant. Don't try the GE silicone II, or any others that say pure, or 100% silicone, they have anti-fungals in them. These are good if you are caulking your bathroom, but toxic to fish.