You should not use pool salt for aquarium purposes. There are a lot of variables in pool salts, and many commercial brands may have additives. Some brands may even contain salts other than sodium chloride. While there are several different uses for salt in the aquarium, pool salt is not ideal for any of them.
Pool salt has no place in the aquarium. While some brands may contain the correct type of salts for aquariums, it's a wiser course of action to select salts you know are safe. Many pool salts contain additives designed to keep the salt from clumping, which can cause issues with water chemistry and may poison fish. Some pool salt is not even sodium chloride, but bromo chloride. While this substitution works fine for swimming pools, it is toxic to aquarium fish. To stay on the safe side, only use proven types of salt for your aquarium.
Marine aquariums work best with commercial salt mix. Salt mix is a premade mixture of sodium chloride and other minerals that simulates seawater. The manufacturers designed these products to be used with purified water, like reverse osmosis or distilled water. These products can safely create water for marine organisms, including both fish and invertebrates. For a marine fish tank, this is the only salt you should ever use.
Freshwater and Brackish Salts
Freshwater aquariums may need salt for a variety of purposes. You can use a small amount of salt as a tonic for freshwater fish. Additionally, salt dips can cure certain parasitic infections. For these purposes, you will want to use kosher salt or aquarium salt. These products contain pure sodium chloride with very little in the way of additives and impurities. For brackish-water aquariums -- aquariums with more salt than freshwater, but less than full-strength seawater -- you have the option of either using freshwater salts or marine salt mix.
Table and Rock Salt
Table and rock salts may also cause problems for aquariums. Table salt may contain excessive silicates, as well as iodine. These additives can cause issues for aquarium fish. Similarly, rock salt may contain various impurities. However, various experts make different recommendations on table and rock salts. When this is the case, it usually means that the experts have had different experiences, and it could mean that some brands -- or even batches -- of a product work under certain circumstances. To play it safe, only use aquarium salt products or kosher salt.