Hi:
Can you give a better description? is the fin torn or ripped by fighting? or is its cloudy, w/ a tattered look to it? This could be one of two problems, but its most likely some sort of a bacterial infection.
I'm not as well versed w/ FW medications, but i suspect a good antibiotic along the lines of a maracyin or nitrofurazan might be useful(don't quote me on the meds-again I'm not as well versed on these).
Also whats the water quality liek these fish are usually in some fairly harsh water condiions.
my opinion
frank