Oh I'M SO GLAD! I've been thinking about this guy and wondering how he was doing. That's so great.
As for why it works, LOL, I don't know the scientific explanation behind it. However, my mom used to use it in a foot bath before she gave her customers pedicures, and even for herself when she got home from standing all day, to relieve swelling in the ankles and feet. As for constipation, I don't know of any human who's actually used it for that, but the back of the bag says you can. I'm guessing it's doing the same thing for the fish that it does for humans.
I use it as part of a salt mix which keeps my pH and KH elevated and steady, so all of my tanks have Epsom salt at 1 tablespoon per 20 gallons, anyway. It obviously doesn't hurt them at all. Here's a link to how and why I do that, in case anyone is interested:
http://www.wetwebmedia.com/FWsubwebindex/fwh2oquality.htm
Again, I'm so glad that this worked. It doesn't even sound like you'll need to medicate with anything else.
LOL here's a link to the Epsom salt council so you can see all the stuff Epsom salt is supposed to do, if anyone interested:
http://www.epsomsaltcouncil.org/about_epsom_salt.cfm
(I don't see its benefits for fish anywhere, but maybe I missed them.)