TotalBoat Thixo is designed to completely cure to an inert material, however it is not recommended for use in an aquarium. Thixo is designed for strong bonds, and can be used in underwater applications, but it may not be safe for some plants or animals if there is any inconsistency in the mix of the two components.
There are many other adhesives that are designed specifically to be used safely in aquariums, ranging from silicones, cyanoacrylates, and other pre-mixed epoxy putty sticks.