what you are seeing is called (HITH) Hole in The Head. It's a diesease that effects fish that are-
A.- Kept in bad water quality filled environments. (need more and/or better filtration + WC's
B.- Stress related do to being chased and harrased by other fish.
C.- Diet (feeding feeders or fish that have not been quarentined properly can contribute to the diesease.
assorted methods used to treat- More filtration, WC's (large), Melafix, Warmer water, Salt.
I've always recommended that a 6' x 18" footprint is the minimum size tank for a large male Jag. We should make this pic a sticky to show people who say a 75 is fine. Look at him in there with no room to exercise his big Managuense muscles. You should definitely either set up your larger tank and commit to giving these fish a proper home or let someone take them off your hands who is willing to do so.