Make sure you have fish that all will do well with salt first. You should get aquarium salt if you do find out it will work with all your fish. The theory is that the salt neutralizes the effects of nitrates on fish somewhat. It's not necessary, but for certain fish, like Mollies, you'll want immaculately clean water without it. Other fish and animals, such as shrimp, snails, and even plants, will not do well with salt at all.