That doesn't looks like fin rot. It looks like biting/injury. You will not see a fish bite. They are sneaky about it. I've NEVER seen one of my biters bite their tail.
Fin rot can start because of biting. That is usually related to poor water quality.
Since it's gotten worse since you moved him into the .5 g. I would move him back into his regular tank. Then start to play around with different things to see if you can discourage biting. Sometimes they want to be in a higher traffic area, sometimes a lower traffic area, sometimes different decorations will make them stop, etc.. I've found luck with Stress Coat + and IAL.
ETA: Don't use any medication or salt. If you want to add Stress Coat + and IAL that's fine. But, medications/salt are hard on their bodies and neither are necessary at this point. For future, I would recommend asking on here before trying to treat. The treatments you already did, weren't really necessary and just put his body through stress.
Last edited by NeptunesMom; 10-16-2013 at 10:18 AM.