Awww! Paul is a sweetie! I'm certain he'll recover fully under your care. I don't really know much about loaches that aren't squirmy, but I agree with what's been said here already. For ANY fish, the very best 'medicine' is clean water and a stress-free environment. It sounds like you've been really conscientious about keeping parameters spot-on, and that is truly the best thing that you can do. There are medications, such as Pimafix and Melafix, that can help to speed healing and be used as a preventative to keep nasty things like fungus from setting into a wound, these ARE safe for loaches, but I don't think it's necessary here. From what I can tell in the image, the 'wound' is clean - it looks like it will heal up very well with no intervention. As you said, dumping chemicals into the water without very good reason isn't usually a good idea, especially when you're dealing with scaleless fish like loaches. ANY med, even 'safe' ones, will add to a fish's stress, and poor Paul has surely had his fair share of stress between shipping to the shop, being in the shop, getting from the shop to your house, as well as from who or whatever caused the injury in the first place. The only thing that *I* would do in this situation that hasn't been mentioned (that I saw) is to limit the amount of time that your tank gets light for a week or so. Loaches, being nocturnal, do prefer lower light, and your plants can do well short-term with a minimum of 4-hours a day. Even if you don't go that route, it looks as if you've put in enough places for him to hide from the light, should he want to, and he's EATING - which is a really good sign that he isn't so stressed. I have found the behavior that you noted earlier to be very common with the loaches I keep - when put into a new environment, they go batty, and are very active for the first few days as they adjust to their new surroundings and water parameters, etc. After that, they tend to settle down a bit. I'm not familiar enough with this breed to be able to say what 'normal' is to them, but loaches in general are notorious little hiders, so I suspect he's just doing his thing. Coloration is one of those odd things, too - just keep an eye on it, as you already are, and you'll know if something is amiss.
It always makes me so happy to see people taking such good care of their wet pets :-D Your tank looks stunning, by the way! Do you have a thread up somewhere where I can see more??!
Congratulations again on the new baby! He's really a beauty. Hope that fin heals up double-quick!
ETA, just realized how oldish this thread is. . . so disregard all of THAT, now I need an update! Is Paul on the mend yet?!!