Monkey~Brain, you shouldn't condone putting a fish in there to get torn to pieces....
Feeding them more often, and lowering your temperature are both good ideas. But instead of removing your driftwood, either put it off to one side, or if it's small enough, put it toward the back further. however, don't remove it, as this breaks the line of sight, and is an advantage to your weaker piranhas. Same flys for the plants.
Add some places to hide. Flower pots turned on their side work well, since I only have a single, I built him a cave out of flagstones. He hides in there most of the time, so if you want big bad piranhas that'll try to eat you through the glass, you may want to discount that suggestion (even though they're more aggressive in groups)
I wouldn't bother with the melafix, not if it's just little fin nips