Never use ordinary table salt (e.g. Morton's) in the aquarium. It contains iodine as a nutritional supplement to prevent goiters in people. Iodine is extremely toxic to inverts and it's not good for FW
fish in large amounts either. Table salt also contains additives to prevent clumping that may mess up the pH.
Marine salt contains large amounts of Calcium, Magnesium, Iodine, and Bromine in addition to NaCl. Good for marine critters - very bad for FW
fish and plants.
Aquarium salt and Kosher salt are essentially the same thing: highly pure NaCl crystals. This is the salt that used therapeutically in FW
Then, of course, there is Epsom salt (magnesium sulfate). Some people use this to treat dropsy, bloat or constipation in FW
African rift lake cichlids require special salt mixes that replicate the hardness and mineral profile of their native habitats.