That form of magnesium sulfate should be fine. As for your fish, continue treatment for a few more days & with any luck that will clean him out. Typically the waste excreted by a fish infested with Spironucleus/Hexamita will be clear, not so much white.
I wish I could offer more for those in need of help with their sick fish, but I don't own a crystal ball & without a proper diagnosis from someone qualified to do so, as in examining a sample under a microscope, we seldom even know with 100% certainty exactly what we are dealing with. Add to that what people are feeding, the manner in which they are treating their fish, including environmental conditions the fish is being treated under, and it's impossible for me to know how each person should proceed. Sometimes no amount of medication will turn a fish around, even when diagnosed properly & treated with known cures. Unfortunately there are no guarantees when it comes to treating sick fish.
I wish I could offer more for those in need of help with their sick fish, but I don't own a crystal ball & without a proper diagnosis from someone qualified to do so, as in examining a sample under a microscope, we seldom even know with 100% certainty exactly what we are dealing with. Add to that what people are feeding, the manner in which they are treating their fish, including environmental conditions the fish is being treated under, and it's impossible for me to know how each person should proceed. Sometimes no amount of medication will turn a fish around, even when diagnosed properly & treated with known cures. Unfortunately there are no guarantees when it comes to treating sick fish.