BaBeL FiSh wrote,
his back-side seems to be (what I can only
describe as) "rotting away". A little of his tail is missing from around
there and he has what seems to be an injury or chunk taken out of his
back-side. ...

If another fish didn't do it, it could be internal parasites or fungus.
Most likely fungus - would treat for that along with an open wound
treatment on the area. Add a fungus treatment to the water after a 20%
gravel vac/water change. When treating an open wound, using a Q-tip
clean the area with peroxide (keeping it away from the eyes and gills)
once a day. Twice a day, rub a little neosporia cream into the area. If
you know a vet, Panalog works a little better/faster than neosporia
cream. Feeding a medicated food wouldn't hurt eather........... Frank