By the looks of that fish I doubt that it's a simple case of constipation, unless it ate a golf ball. lol
If in fact it is a case of constipation, I wouldn't recommend feeding it epsom soaked pellets as outlined in my sticky. Adding more food to a constipated fish can sometimes result in an internal bacterial infection.
While epsom salt can act as a purgative agent, that treatment was designed for a very specific pathogen, not to relieve constipation. In severe cases of constipation I wouldn't recommend feeding anything, but epsom salt can be used as a bath treatment, 2-3 teaspoons per 5 gallons of tank water usually causes a movement in most fish within 48 hrs. After that I would continue to withold food for a few more days, then ease back into things feeding sparingly for several days while the fish has time to adjust and recover.
HTH