If it is just a salt mix and doesn't contain any vitamins or any other additives then there is nothing in it to go bad. Everything is stable inorganic salts which do not decompose.

How long does aquarium salt last?

We've kept and sold thousands of fish at our fish store, and this level of salt is safe for virtually all fish – except anchor catfish. Keep the fish in the salt solution for 4 to 5 days, and then increase the concentration if there's no improvement.

Does saltwater aquarium salt expire?

salt doesnt expire. if your tank is mature with mature corals growing in it..