silicone

Depends on what you're holding together. Silicone works great for glass to glass applications, but doesn't work well on plexiglass or acrylic. Marine grade epoxy will work better for that. You can also get acrylic adhesive--some LFS carry it, and it works very well. For silicone, use one that is pure silicone, and doesn't contain any mildew or algae inhibitors. Most home-repair type places will carry a brand that works; here in the States, Dap has a brand that is aquarium safe.