He wasn't lethargic in the store. I noticed the darting maybe ten minutes or so after putting him in the tank. He was banging into a bunch of things so he could have been rubbing against them. I moved him to a 2.5 gallon bowl so I could bring him home for the weekend. It is heated and last I checked it was ~77-78. I read a sticky about external parasites and the description of symptoms seemed to fit so I bought malachite green and started treating him with that.
I can't really tell if this behavior has changed since he was in the store. I mean, he wasn't darting around his cup, but he didn't dart or anything when I cupped him, either. I think the behavior started because I finally had enough room to move around.
Right now, I wouldn't call him lethargic. He's swimming around and still darting (or is flashing the right word?). His fins are a bit torn now, but that's because he jumped out of his cup when I took the lid off and was about to move him into the bowl. Other than that, his appearance hasn't changed at all. I have to go to work in a little while, but I can try to get some pictures of him later. Though, I don't know how useful they'll be for diagnosing since all I have is the camera on my phone.
I also have aquarium salt and epsom salt. I thought about adding some aquarium salt for his fins, but I wasn't sure if it would have some sort of reaction with the malachite green, so I decided against that.