I've never seen the allergic reaction that Cerianthus describes, and certainly that could be it, but when I do a close-up on the loach on that last photo I can see what looks just like a dusting of flour, tiny tiny specks of dust on him, especially on his head and face down towards his nose and mouth.
If you did use MG in the tank he's in you could run some carbon in the filter for a couple of hours to get any residue of MG out of the water column, but I would start with an anti-parasite treatment ASAP.
The medication that Tonergirl mentioned is a good one, and I called and talked to a person at Jungle Labs and they said that Jungle Labs Parasite Clear should take care of it, you just do a half dose for Loaches because they are sensitive.
Any anti-parasitic treatment may need to be done at half dose in the case of the loaches.
Cerianthus is right that your loach looks nice and fat; it's obvious you've been taking good care of him. I would respectfully suggest, however, that you seem over stocked from the looks of the video, and from what I've learned CLs need a huge tank and really need to have more of their kind to be comfortable and happy, so he may be a bit stressed in this tank.
I would strongly recommend that you get a good liquid test kit, such as the API Master so you can carefully monitor water quality. Parasitic infestations can often often be resisted or fought off by fish with healthy immune systems if you're water quality is pristine, though the new additions to the tank probably did introduce the problem.
You need to keep your parameters at: ammonia 0ppm, nitrite 0ppm, and the nitrate at 20ppm or less. A lot of times, in a cycled tank, it's the nitrate that has slowly crept up to levels that are harmful.
You may be keeping the water really pure, I didin't read your original thread so someone may have asked, but how often do you usually do water changes in that tank; frequency and volume of water removed and replaced?
What water conditioner do you use? I recommmed that you get Prime by Seachem, especially will this be helpful while you're trying to treat the CL and others in your tank. Prime can help by detoxifying the ammonia, nitrite, and even helps with nitrate as well. While using the anti-parasitic you many need to hold off on water changes for a few days and so the Prime can help keep them safe while traces of ammonia or the other harmful by-products are in the tank.
ADDENDUM: Cerianthus' recommendation of Clout is good as well, it's a good product, and I'd prefer that over Copper safe, and I think it's AquaZole by Seachem that you're talking about Cerianthus. I posted regarding this in the other thread on this case.
Nangus, you need to try to stay with one thread or the other regarding this problem so you and we won't need to be going back and forth to try to keep up with your questions and the input of others.

I'm not meaning to be critical, please know that. I know how deserately you want to save you CL, and the others.
It's in the other thread that has the best photo of your CL showing a close up of his head and the dusting on him.