It looks like either it's fins were picked on by a tankmate or it looks like Fin Rot.It even could be high ammonia levels in your tank causing Ammonia poisoning. I would get a API dropper Master test kit to test your water. The test strips are not that accurate. Ammonia spike can be fixed with water changes.
I use this stuff by API called Melafix and also Pimafix that you can get at any fish store. Use together for quicker wound healing.
PimaFix - is a natural botanical remedy for fungal and bacterial infections/ Harmless to aquatic plants/Safe for freshwater and saltwater aquariums. This treats cotton-like fungal infections and both internal and external bacterial infections.
Melafix- Antibacterial remedy is an excellent alternative treatment for resistant strains of bacteria unaffected by traditional medications. Treats bacteria infections as little as 4 days and heals wounds,ulcers, and damaged fins. Does'nt affect Ph, is safe for invertebrates like snails or dwarf shrimps. I have even used this with baby fry in my tank that tells you how safe this is. I like keeping this medication on hand at all the times.
I found a web-site that lists fish diseases that might help you also: http://www.fishdeals.com/fish_diseases/fin_rot/
If you click on the Symptoms of Stress & Disease on the Fin Rot page their is a huge page that has other fish diseases and problems to pin point the problem you may have.