Is it permissible to sell and buy salt used in aquarium tanks to turn the water into saltwater, and what is the ruling on trading in other aquarium products?

There is no harm in selling and buying salt used in aquarium fish tanks, as long as it does not harm the fish. There is also no harm in trading in aquarium products, for the original ruling concerning sales is permissibility, as Allah Almighty says: (And Allah has permitted trade).