Whiskers, I think you're thinking of aquarium salt, which btw, thanatos, is unnecessary in freshwater tanks. (Don't buy it... haha) Epsom salts is supposed to be used in a hospital tank in cases of dropsy, as it is said to help draw excess fluids out and also to be a fish laxative.
As Orion says, it can also be used in homemade buffering salt mixes, which are for specific uses in cichlid tanks. In a mixed tank such as yours, thanatos, using it as such would be a mistake.
You really, really need to stick to basics for now. Later, once you learn how to keep a tank going without any catastrophes, there will be plenty of time to explore all the advanced areas of fishkeeping. Babysteps, thanatos. Babysteps.