What kind of adhesive do you use on a fish tank?

For building decorations and aquariums, I use pure clear silicone sealant which is also labeled "For Aquarium Use". Do not use silicone with any kind of algaecide in it (such as Silicone II at the hardware store) as the algaecide is toxic. Let the item dry until you don't smell fumes anymore and then it is safe to put into the tank or to fill with water. I also use aquarium epoxy (cut piece, knead, use), but it only comes in "stone" or "white" and both colors look like Chewing Gum in the aquarium. It dries rock solid, however, and is what I use when I will be able to completely hide the epoxied area. It works great for sealing pipe leaks and fittings and for building sumps.