Melafix/Pimafix products have a lot of fans, but some people with bettas are leery of possible damage to the labyrinth organ so I believe using them at 1/2 strength would be recommended for Fishy. (In fact, there's a product called Bettafix that is just a less concentrated version of one of the two ... Melafix, I think.) FYI, they are more like antisceptics than medications so are helpful at cleaning up minor problems but can't cure serious illness. If Fishy's problem is water quality, then using these products with daily water changes could do the trick — and, like Wendy pointed out, won't hurt filter bacteria, so all decor, gravel, etc. can go back into the tank (to start rebuilding that bacterial colony).
Janet, you should be able to vacuum the gravel with any old siphon, just stick the end of it deep into the into the gravel during water changes to draw out the gunk when you're removing water from the tank. You can vacuum just part of the gravel with each WC, a different section each week.
BTW, I sort of love that you call your fish Fishy. I have a male betta boy that I keep calling Betta Boy. He's liable to get stuck with that as his name unless I have a sudden genius name-the-fish moment in the near future ... which is not so very likely.