I just did. ^_^
And I don't think you should use pimafix, I think it's bad for bettas long term. Don't worry, he should be okay as long as you stop using it now. If I am wrong, I hope someone will correct me but that is what I remember at the moment. I am not an expert in rescuing fish yet, but since it's bare and he's in a new place, you could try put one plant in it for him to hide in. If he's a sick fish, stress won't really help, they like to feel safe. :)
Try using aquarium salt (API) for ten days, and only half the dosage on the box....or 1 teaspoon and replace with every water change.
You can try getting indian almond leaves, that helps damaged fins heal quicker and it's naturally what wild bettas have. If that's not possible, you could get betta water conditioner by Aqueon, it has almond leaf extract in it and works really well too in aiding their health. They are very small bottles, so it's better to use them when your fish get sick, otherwise regular conditioner. Of course keeping the water clean is a major part, but you probably know that. xD
Hopefully others can tell you of other medications to use, but one that's good for bettas is Hikari betta revive health. I saw it at petcos site, apparently it helps alot of different health problems.
^ Assuming it's good for them because it is made specifically for bettas. And Hikari makes pretty good products! :)