I don't have experience with fin rot and his fins only look incredibly chewed up, not like they're dissolving, so I'm not sure whether to treat him for it or not. I don't know if it would stress him out more or not doing daily 100% water changes. He seems like he's doing good today. He's eaten three pellets this morning and his little belly looks nice and plump. Gills are moving normally, although sometimes it looks like he's breathing heavy. He really tries to move around but with his tail shredded he seems to have a difficult time. If someone recommends doing a 100% WC later then I'll probably dose the tank up with methylene blue.
I would keep the water as clean as possible. Definitely change it 100% every day! That's the best thing for messed up fins, just clean water and he'll heal right up. A little aquarium salt might help, but clean water is the best medicine. You did the right thing in taking him home. He's a cutie!
He is very slow-going today. He explores on occasion but otherwise just stays towards the bottom. Currently no signs of dropsy, ich, velvet.... Keeping an eye on his fins though, if they're not better tomorrow when I get home I'm going to get something for fin rot. I'm going to dose up the water with methylene blue.
Edit: He's in a little less than 2 gallons of water at the moment. Methylene blue recommend dosage per gallon is 10 drops, added 4 drops of methylene blue to his water. Monitoring gill movement, activity, etc.
He's a beautiful fish, I'm just hoping I can fix the damage that has been done. I'm using the methylene blue to help with any diseases he might have captured or been close to capturing, as well as treating him for poisoning of any kind from his dirty store water. I'm very nervous at the moment. His gill movement is still normal, he's moving his little front fins rapidly to show he's still alive. Otherwise he sits in the bottom corner of the tank, occasionally moving his torn tail. I wonder if he's afraid of the vast size of the tank, coming from his bowl, but its the only tank that I can heat to about 78-80 degrees and its not even full all the way so he won't struggle for air.
I'm hoping OFL or someone else knowledgeable can lend me their brain for some information regarding what the next step should be with this little guy. My nerves are wracked with him. @.@ He ate three pellets this morning, still awaiting the poo from that.
Updated pictures. I moved him down onto my printer beside Shif's tank (Shif is really ticked off >_>) and he seems more active, exploring. Maybe he just didn't want to move, surrounded on 3 sides by crap and a wall...