I have both good news and... neutral(?) news.
Good news: He was getting worse in the 3/4 gallon floating tank, so I slowly acclimated him to tank water and put him back in the 5 gallon tank. I've been gone for about 9 hours, so he's been in relative darkness and peace and quiet during that time and he's perked up a bit! He was swimming around when I snuck in and his fins aren't clamped hardly at all. He just in general looks better than he did.
Neutral news: Another something I'm not sure if I should be worried about... his belly looks a little swollen. Nothing terrible, just kind of like he had a big meal - but he hasn't eaten anything save for about 4 of the NLS small fish formula pellets this morning (tiny little things). The small amount he ate wouldn't affect him like that. Maybe it's just because he hasn't had a poo in a couple of days? I'm going to hold off on feeding him, let him sit overnight and check again in the morning.
Feel free to chime in with any thoughts/concerns you might have.