I heard the Epsom salts is mainly for freshwater fish. I want to try it but don’t want to continue to make my parameters unstable
Both the peas idea and the Epsom salts are from freshwater fish cures, specifically goldfish. These got over-extrapolated to marine fish.
The person who came up with the peas idea worked for the Shedd Aquarium, it was designed for gas in the intestines of fancy goldfish (they get this due to their malformed bodies). Marine fish like tangs normally eat nori, and so do not have any issues with lack of fiber. It has nothing to do with the fish's swim bladder.
Epsom salts, magnesium sulfate, is the fourth most common salt in our sea salt mixes. Adding a bit more to a marine aquarium has no benefit other than to raise the salinity a bit and skew the ionic concentration.
Jay