No sure about that, I think normal aquarium silicone does have something in it that will not be good for your fish, only when it is completely curred its aquarium save
If you are re-seaming a seam or similar, the tank needs to be empty and completely dry. You need high modulus silicone; the type you see in the diy shops usually are low modulus, and contain anti-fungicidal additives, which are toxic to fish. As far as I know, there isn't a type that can be used underwater.