Platy Swimming Straight Down After Levamisole

My new (as of 3 weeks ago) guppy started getting emaciated and swimming funny. Too late, I figured out it was the camallanus worm (unmistakable- he had the red spider-like worms peeking out of his anal vent). I did a lot of thorough online research and ordered Levamisole from Subquaria to treat the tank (because if one fish has it, all will end up with it, according to the internet). My guppy died the day before the meds arrived (yesterday), unfortunately, but when the meds came in I dosed the tank anyways (no other fish were exhibiting symptoms of camallanus or any other illness) to kill off any remaining parasites. I followed the directions to a T, which involved doing a 50% water change (the water I added was treated with the same dechlorinator and some aquarium salt, which is how I always do it), removing activated carbon from filter, and adding the proper dose of Levamisole.

Now, an hour later, I went to check on my fish to make sure there weren't any ill reactions to the meds... and I found my platy swimming straight down, unable to straighten out. She levels out when she propels forward, but as soon as she is still she is vertical again. No other symptoms, fins not clamped while she swims, scales aren't sticking out (like dropsy), nothing. She was healthy before this. All other fish appear perfectly fine.

Any ideas what the heck happened and how to treat???

Specs:
-20 gallon tank
-2 platy, 1 guppy, 3 red tetra, 2 cory catfish
-78 degrees
-nitrate: 20ppm
-nitrite: 0ppm
-ammonia: 0ppm
-pH: 7.2
-kH: 80
-GH: 120

and, additional spec: I've had this tank for a year and a half, it's fully cycled. This particular platy was born in the tank and is 10 months old and has had no health problems til now.