The general concensus is that the tea tree oil in bettafix will damage the laberynth organ - which is what they use to breathe from the surface. Others say it's a miracle drug. I have used it as I had nothing else avalible to me and saw no negative side effects. However, it's probably best not to use it..
If constipation is the issue - stop feeding him and use the Epsom salt. You can go up to 3 teaspoons per gallon if needed but you should slowly work your way up to that amount. (do 1 teaspoon per gallon then several hours later change the water and replace it with 2 teaspoons per gallon and then several hours after that change the water and replace with 3 teaspoons per gallon). Daphnia is supposed to be good for constipation but not sure where you would get it from.
A constipated betta will have an unusually swollen abdomen but in all other rights appear and act normally.
Does his stomach look llike this
are the scales sticking out like this? if so, it's Dropsy and not constipation