galls tank or acrylic. If glass, I would suggest using silicone specifically branded for use in aquariums, many other sorts contain fungicides or bacteriacides in them, bad bad for reef critters. The aquarium specific stuff costs more, but worth it when you weigh its cost against the cost of the life you want to keep in your tank and the time spent nurturing it. I have no experience in sealing acrylic, generally these are permanently bonded together at the seams.