Clown Loach: Euthanize or Treat?

Hi everyone, my tank has been set up for awhile now, and I had introduced some small clown loaches from Petsmart about a year ago. Surprisingly, the entire school was fine and never displayed any issues.

Over the past few months, I've seen less of the Loaches. They typically just hide out in their driftwood caves and I can hear them feeding when the lights are off. Within the past week, I started to see one swimming around, which was very uncharacteristic of it. I brushed it off until it started to sit on top of my one powerhead on the other side of the tank form where he would normally chill.

I saw a small little bump on its side and thought it might have just poked itself on the driftwood. This was about 2 days ago, now it seems a lot worse and the poor fish seems very unresponsive to its surroundings. I tried to hold food out in front of it but it wouldn't eat... which has me extra concerned now. I moved him into another tank I had set up mainly as a plant hospital tank but I fear its condition is worsening.

To help provide information about the tank it came from (based on the submission guide)

1. The fish having a problem is a Clown Loach. Has bumps on one side. Sluggish behavior

2. Reading 0s for Ammonia, Nitrites, Nitrates. Temperature is around 79F.

3. Tank is a 125G long. Been set up for a number of years, can't remember exactly. Back in September or so I redecorated the tank.

4. I run a Fluval FX6, which is like 563 GPH or something. I also have 2 Koralia Nano powerheads for additional movement and an AquaClear 70 Powerhead with the Quick filter on it (positioned on the other side of the tank from the Fluval's intake) and a custom spray bar.

5. Stocking in the tank is a school of smaller Corydoras (about 8?), some Glowlight Tetras, some Celebes Rainbows, 2 Clown Plecos, the 4 Clown Loaches, 2 Flying Foxes, and a handful of Panda Garras. All except for about half the Panda Garras have been around since before the Clown Loaches were introduced. Nothing really bothers each other, the Flying Foxes used to chase each other but now chill together.

6. The "sick" Loach is about a year old.

7. I recently moved around some plants and transferred between the plant hospital and main tank. The plant hospital had only a small school of tetras, which are exhibiting no ill effects. Those tetras came from the main tank around September during the redesign and I've owned them longer than the Loach.

The first picture in the link shows the issue. The second shows the other side (the tank has a little dust on the outside. There aren't white specks on the fish, those are on the outside of the tank)

http://imgur.com/a/ruKR1

I've got a few different medicines on hand but I don't even know what the issue is, let alone how to treat it. Anyone able to help? Thanks in advanced!

EDIT: I feed my tank a combination of Tetra Flake and Crisp food, Hikari Sinking Carnivore Pellets and Hikari Algae Wafers. Frozen blood worms are an occasional treat. I've never seen any problems with food being unfinished or not eaten at all by something in the tank. I generally only feed once a day shortly before the lights go out. This has been my feeding schedule for well over a year.

EDIT2: been a little over an hour. I left the loach alone with a towel over the tank to block out light. It seems to be swimming around but none of the food has been touched, despite it sitting with its mouth a few hairs from one of the pellets. The partial algae wafer was a little distance away and no response either. I'll check again in a few hours and see if there has been any interest.

EDIT3: Left for work this morning and it was casually swimming around again, but it wouldn't close its mouth and its gills were rapidly moving. Came home and it was stiff as a board :(

It looked like it developed more bumps in the hospital tank. I'm in the process of increasing my main tank's temperature up a little in case it was Ich. Thanks to everyone that responded. This was my first major illness in well over a year so I started to panic.