PlanetCatfish.com

Just received a rather nice box of fish; amongst them was an L213 male (think the fish might actually be L359, as were ~70%+ of all the "L213" imports last year). Gonna go out on a limb here and say the poor fellow's toast and doesn't stand a chance of survival. I've never had a WC pleco come back from emaciation this severe; extremely sunken eyes and hollow belly were never a good sign IME. Internal parasites and resulting stress causing lack of appetite are likely the main culprits, I'm thinking. I've upped the temps on the tank he's going into; did another WC with 50% RO water, and added more alder cones and bark, but I'm hesitant to do anything more- I know meds are necessary, but I'm still holding onto the slightest bit of hope of getting him to eat something.
The fellow's still in a bag as I'm waiting to hear back from the seller, but I'm more likely going to release him into QT soon and just chalk it up to a loss. Bit of a shame, really; they're one of my favourite Ancistrus and I'm still mentally kicking myself for not picking up a few WC adults last year (in much the same manner as when I passed on a group of very large, nicely-coloured WC CW009 selling for $20ea at a Petco three years ago ).

Does it look like the fellow has any chance of survival at all, or is he too far gone? He is rather listless, but not completely immobile.

Update: He's dead. Not literally, yet, but sure will be in two hours. Was much more worse off than I thought. Rather disappointed. Think I still have a potential pair, aside from him, though... Perhaps if they do alright I might at least someday get some F1s.

So sorry, Ancistrus can be a very challenging genus to keep.
Once that skinny it really depends how long they have been like that. It's about getting anything in them but after that the healthy diet. I do suspect there might be a use in having them in with other related fishes who willingly feed if you can't get others of the same species.
It's possible they don't recover even if feeding again or will never reach as good of a health as they could have.
I find it hit and miss which wood produces or encourages the right biofilms that they will eat. I've never found they really utilise those biofilms. I just use Repashy soilent green with added algal powders for long term care but I also find it helps them feed right away. Sometimes with these Ancistrus certain vegetables might work