Hillstream Loach Health Concerns

[Helmet]

Sorry to hear he's no better!

Personally I reckon you could be a bit more robust with your treatment..1 drop per 1 gallon is a very discreet amount I can't imagine it having much effect. I dosed meth blue at 5ml per 50 litres I believe , it won't harm your fish, it will kill your nitrifying bacteria (along with most bad bacteria which is the point) but as you don't have any of them that won't matter. Dissolve the amount of medication in a litre of warmed tank water before you dispense it evenly over the surface of your tank.
Regarding your water changes , I use Seachem Prime in your exact situation as it detoxifies Ammonia/Nitrite/Nitrate as long as you add it every 48 hours. That way you can leave the meds in that bit longer to work. I find that using that along with Stability when medicating keeps your fish safe from toxins resulting from having no bio filtration

His appetite? Well I don't always see my hillstreams eating as much as other fish, however they are healthy & active. Just because you don't observe him eat in the time you're watching doesn't mean he isn't eating .. If you follow me.. How is he acting apart from that? Any rapid gill movement , lethargy etc?

Good luck