Yeah, I posted a thread similar to this a little while ago; it's in the Betta/Gourami Discussion if you look for it. It's called I think "Betta's tail not growing back". Anyway, everyone said that Melafix was the best way to get any regrowth, and that it does it quickly. They also warned that you must be careful with it's concentration. Apparently, the concentration of Melafix is much higher than that of Bettafix. The Melafix's high concentration of tea tree oil will actually burn the labirynth organ of the betta. Basically, don't waste your money on a teenie bottle of Bettafix, just buy Melafix and use much less of it than is prescribed so that in your water you end up with a lower concentration. If you look in my thread, there is an exact "recipe".
I just started treating my fish two days ago, so I'll see if his tail will grow back. His just mysteriously shredded by itself. Good luck!!