Water softener salt is typically made of sodium chloride (NaCl), which is also the main component of table salt. However, water softener salt also contains other minerals and chemicals that can be harmful to marine life, such as magnesium, calcium, and potassium.
These minerals and chemicals can alter the water chemistry in your marine tank, which can stress or even kill your fish and invertebrates. In addition, water softener salt can also interfere with the proper functioning of your aquarium filter and other equipment.
For these reasons, it is important to use only pure, marine-grade salt in your marine tank. Marine-grade salt is specifically formulated for use in saltwater aquariums, and it does not contain any harmful minerals or chemicals.