what kind of substrate? have you added anything to the tank recently? any source of carbonate (like seashells or the like).
some wood will also lower ph...just dont soak it for ages (ie-let it still have tannins for the water...will lower ph gradually). dont use gimicky stuff like pH down-causes more confusion and params will jump all over the place while you try to adjust the ph.
how long did you acclimate her for? with slow,careful acclimation the pH shouldn't really affect her all that much. she might be stressed over the huge space she has all to herself-esp if she was used to being in with other fishies (even if they DID munch her tail to nothing). could she be having trouble with the filter flow (hence her wobbling..without the majority of her finnage, i would imagine it would be hard to maintain ballance with ANY current, even if tiny).
keeping her isolated for a few weeks is a good idea. her fins will most likely grow back, even if completely missing. clean water (so lots of changes). once she is a little "stronger" i would put her back in the main tank.
best of luck with her-she's a cutie pie.
cheers