Here's an extract from an article I wrote - does it look like this?
Thanks jolliolli for the pic...
Fin Rot
Fin Rot is usually a symptom of illness rather than an illness per se, although in some cases it can result from direct bacterial or fungal infection.
What it looks like:
Fin Rot begins at the edge of the fins (or at an injury site) and progresses rapidly toward the fin base. It results in a ragged 'torn' fin edge often with a whitish edge (dying / dead flesh). Once it reaches the base of the fin, the fish will not be able to regrow the fin, and infection may spread into the fishes body.
What causes it:
Fin Rot can be caused by physical injury, poor water conditions, poor diet, general stress, or appear in connection with a primary general systemic infection (usually gut infections). It is most often seen in weakened fish.
What to do:
Because it is a symptom, it is important to identify the likely cause before an appropriate treatement can begin. Small infections will often heal without further treatemnt if the cause is found and remedied. The most likely causes are poor water or gut infections. Test water parameters and rectify where necessary. Look for signs of gut infections (see below) and if found treat the gut infection with an appropriate drug. If these causes can be ruled out and there is no other obvious cause, or if it progresses very quickly, treat with Furan2.