Very Poorly Pleco

Hey, I'm not sure what to do regarding my pleco. He seems very thin and drained of his old black colour. He's been like this for a while and I just don't know what to do. I've put in fin rot medicine as I looked up his symptoms and it sort of matched that but maybe that's not it. Plus, all my other fish seem fine. I have a 300L and have 5 clown loaches, 1 albino ruby shark, 8 leopard danios, 1 opaline gourmai and the pleco and everybody but him, are doing great and thriving. I do a 25% water change every week and the fish always seem to appreciate that and aside from some worming problems with the loaches earlier this year, I've had no problems.
 
I know it could be a sign of bad nutrition, but I don't know what to do about that. I put in a variety of foods: flakes, vegetables, frozen food, I even brought some algae wafers just for him, but as soon as I put in anything, the loaches are on it instantly and I sometimes wonder if he gets enough, but I do my best to try and put in extra so he has a good shot. He still has interest in eating. He always comes out to eat when I put food in. 
 
Here's some pictures I managed to take with my phone. (The stuff on the gravel is flakes. It's feeding time.) The quality's not great, but it's the best I can do.
 

 

 

 
 
Any help will be appreciated. I'm very worried.

drop in an algae wafer just before lights out---plecos roam about more at night

Hmm, I am not a fish doctor, however i've heard of "mouth rot" and I've just googled "Pleco Mouth Fungus" and pictures have come up similar. If it's the one fish being affected, I would set up a quarantine tank for him. Google mouth fungus, and see if this seems to match his condition. I hope this helps you