Hi,
Epsom salt is used as a laxative for fish. First step when bloating (swollen belly) is noticed. 1 to 2 tblsp/10 gal.
Other salt, noniodized table salt, aquarium salt, rock salt etc
is used to help the fish breath better (makes gills work better) at 1 to 2 tblsp/10 gal. This salt can also be used as an mild antibacterial/antifungal at up to 1 tablespoon per 1 gallon water.
hth