Actually I think he had internal and external injuries
Yeah I treated him with medicine for internal injuries as
well
He only eats deshelled prawns
Anyway he is back to black though still not completely stable
coz he still have a golden sheen
Nurse? That would be me >.<
I had a carp once that grew to be large and fat although when I
got him, he was just a small fry. One day, I came home from
school to notice that he had reddish wounds on his sides.
Didn't know why until when I did my homework in front of the tank,
I saw him suddenly go berserk and just make some kinda mad dash in
the tank, slamming himself against the walls of the tank like
that. 
I think that might be what your Gulper had been doing. I
think maybe the tank is a tad too small for them and they are
ambitious fish who need more stimulation in their environment or
something... and they went postal, thus sustaining internal
injuries.
I think you need to consult a Fish Psychiatrist.
