I ran into the same problem when trying to add to my shoal of Kuhli - they were VERY scarce around here all last year. They finally started coming in, but the shops (and many members here, and on other forums) were all complaining of 'bad batches' in which the entire shipment would die. It's really sad. :( Now I'm seeing some healthy ones finally - and I have no QT space for them! I hope I can find a few more healthy ones in a few months when I'm ready for them :/ Hope you find some more, too - if you're looking still. . . to the best of my understanding ALL Kuhli found in shops are wild caught. I've read reports of them breeding in captivity, but so far it always seems to have been kind of accidental. Nobody has been able to intentionally breed them yet, so no farming for Kuhli. Not sure what's going on out there that we've been seeing so many sick ones. . . I don't know that I would be looking for more if I'd experienced something like that, those images are pretty awful - I'd have been a disaster if I had been forced to see it IRL. *hugs* On the other hand, how can you resist Kuhli!???
I REALLY hope your last survivor remains well - and I've seen many tanks in which exactly that did happen. *crosses fingers* VERY HAPPY that you aren't seeing any worms. Fingers firmly crossed that this was the end of it for you. . .
I've never seen BB sized gravel! Whatever it is, you had your original loaches for years - they seem to be fine with it. The biggest concern - aside from those tender bellies - is their super sensitive barbels, which sharper gravels and sands can wear down and erode. Loaches use them to find food, don't do very well without - just something to keep an eye on if you haven't been. . . they can be fine kept over smaller, rounded gravels. Its usually the 'sharper' ones most commonly sold in pet shops that seem to cause issues - but that's only what I've read. :)
If/when you do decide to treat the tank, be very sure to double, tripple check doses for ALL of the animals who live there. Loaches especially have very small scales, and are super sensitive to meds - I don't know about the rest of your fish. . . Please keep us updated on how things go in your tank. . . *worries*