Pleco with red fin tips (no good)

Question
I have a very mature(1') and a mature(8") pleco among six red bellied piranhas(5.5"-6.5") in a 60 gallon tank.  The tip of the oldest pleco's tail fin is turning red.  this happened once before, affecting both plecos, when the water was dirty.  However, the reoccurance is only affecting the oldest pleco and more severly than before.  the water is crystal clear what can it be?

Answer
Hard to tell i found this regarding it:

7) Reddining of the skin, or red streaks in the fins

Virus, Ulcers (Bacterial infection) Viruses can only be seen through microscopes - and its incurable.

With a bacterial disease, use a anti-bacteria or anti-internal treatment. A prolonged SALT bath (3-5 g/litre for several days) will alliviate osmoregulatory stress in the case of severe ulceration.


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Also you can check out this site for fin rot that lists your red fins as a symptom
http://homepages.tesco.net/~davyreynolds/fishhealth/finrot.html