Luis,
You may be facing multiple problems here. Was the fish a long term captive? In one of the shots, it is clear that there is a distinct bulge in its abdomen....not something you typically see in Pygoplites! Either the fish was eating REALLY well, or it has edema from kidney failure.
The skin lesions, cloudy eyes, rapid breathing and fin erosion is probably an advanced Cryptocaryon (ick) but could also be Brookynella, or either of those problems combined with flukes and/or a secondary bacterial infection.
A freshwater dip at this point will not help, esecially if you return it to the same tank. Raising the temperature to combat Cryptocaryon is often incorrectly applied - it must be done in conjunction with a treatment of some sort. The higher temperature is only meant to speed up the life cycle, getting the parasite to the point where a treatment can work on the vunerable tomite stage. Raising the temperature without a treatment will just make the fish sicker sooner.
Sorry to be blunt, but the key to the probem is really your statement, "this was a fine specimen about three weeks ago " - the best chance for action would have been 20 days ago...it seems too late now.
That said, if it were my fish, I would move it to a hospital tank and treat it with chloroquine. Copper is usually my first choice except that it usually takes three days or so to work, and I don't think the fish has that long. Alternately, you could try a 45 minute formalin dip at 200 ppm followed by hyposalinity at a specific gravity of 1.009 - but you'll have to drop the fish to that level in 12 hours or so, and this treatment also may take longer than the fish has left.
Sorry, but IMO you need to watch your remaining fish very closely for simlar problems....
Jay
