): The fish I'm taking care of for a friend was kinda like that. He'd just lay at the bottom of his tank and only moved to get air.
Because he also had fin rot (and suspected internal parasites), I was using Maracyn-Two, but I also did salt baths every day and now he's really energetic compared to what he was like before, although he still rests a lot which I can understand because he's missing half of his tail so swimming must be tough for him... although usually when he goes to rest he sits in the corner and flares at his reflection in the tank now xD;
I don't know, this is just my personal experience with this kind of behavior. ): So in my opinion if he used to be active and now isn't, I'd imagine there's something wrong. If it's internal it'll be a lot harder to figure out :S But don't jump to any conclusions just yet. Maybe do a little research on fish illnesses and keep observing him for a little longer?