it always good to have aquarium salt and epsom salt just as emergency kit. Both of them hve antibacterial/fungul effect. The same instructions for both . But aq salt you can use no longer then 13 days ...the course is 10 days and in some cases you can use 13 days. Epsom salt you can use for long time as much as you need...
Epsom salt you use for buoyancy problem(sbd , pop eye, constipation, internal parasites)
Betta is beautiful i hope he will pull through. Looks good though ..
Give us update. Did you try frozen blood worms?