Sick Fish - peeling lips

I have a goldfish that has been doing ok for over a year. She is about 10"
long. Suddenly she is having problems with the skin on her lips peeling off.
I know it irritates her, as she is having periods of thrashing about, trying
to dislodge the loose pieces. Her slime coat seems ok, if a bit thicker than
normal, and on occasion she will gulp water. The tank is shared with two
male goldfish 1@ 12" and 1@ 7" long, and one female neon rainbow, and 4 male
neon rainbows. The rainbow's are all 2" long or less. They have gotten along
well for over a year now. I lost 3 of the female rainbows a few weeks ago.
They got very weak, and ended up floating nose up at the top of the tank.
Once they got too weak, I had to put them down. It was a shame, but it was
all I could do for them. Nobody seemed to know what was wrong with them. I
think the males tormented them to death trying to mate constantly.
The tank is a 75 gallon, half bare bottom, half natural gravel. It gets
vacuumed once a week. The water is changed out at the rate of 15 gallons
every other day using 3 TBS of salt per 5 gallons. The temperature, Ph,
nitrites, nitrates and ammonia are all within normal levels. The
water/tank/filters had gotten a bit dirty while I was sick a few weeks ago,
but I got it back into condidtion as soon as my doctor would let me do the
lifting.I suspect that she got sick from that, as I normally keep the
tanks/filters very clean. The filters are two magnum 350 units with one
double bio wheel set. I figured she would just heal up once the tank got
back to normal, but even though she is doing much better, her lips are still
bothering her, and have visible signs of enlarged membrane and peeling of
same.
Any ideas on what this may be, and/or how to cure it? I'd hate to lose her,
as she is normally very playful and personable. She won't even come over to
rest in my hand now, and that's not normal for her. She usually comes right
over to be stroked when I clean the tank. And it's not abnormal for her to
let you hand feed her. Now she is still active, but has a definate attitude!