Cyanoacrylate Glue (“Reef Glue”, “Krazy Glue”, “Super Glue”) – These glues are some of the most common, especially among aquarists and reef hobbyists. They are effective with many different types of materials and are very strong, particularly when bonding plastic materials. They work well with reattaching coral frags that may have dislodged or fixing ornaments and they cure quickly. Most of them tend to turn milky-white if they are put into the water while the glue is still wet but they are otherwise safe for lots of applications.
- Silicone Sealant – Silicone sealants are usually used to fix the seams of an aquarium but they can also be used in assembling ornaments and pieces within the aquarium. It is usually available in black or “clear” (usually more milky blue-white, in my experience) and can be thicker that cyanoacrylate glue, but it is durable and more flexible once cured. Be sure to read the directions to make it easier to use and cure it fully before using it in your aquarium.
- Epoxy – Epoxy is a two-part adhesive that needs to be mixed together to activate. Underwater epoxies usually look like a putty with an outer coating over a contrasting center and are commonly found in white or a coralline-algae-colored purple. These epoxies are more cement-like than other adhesives and are good for creating rockwork formations but not as effective for surfaces that need a thinner, more transparent adhesive. Avoid using epoxies that aren’t designed for underwater use or with toxic materials, especially before the epoxy has fully cured.
- Hot Glue Guns – Hot glue guns are arts-and-crafts staples but are also surprisingly effective in aquariums, most especially in freshwater tanks. For quick fixes like re-attaching an artificial plant that may have become detached from a base, they are the easiest to use and are non-toxic and ready to use soon after applying. Make sure the pieces are completely dry and clean and avoid using this glue in higher-temperature tanks.
- Water-soluble glues – For obvious reasons, never use water-soluble glues like white craft glue in aquariums. They will never cure and will affect the water quality.