I'm assuming you didn't quarantine the new loaches. Because they're wild-caught, it's common for them to bring in all kinds of strange diseases.
I recommend treating the tank with Maracyn Plus or another broad-spectrum antibiotic, as it sounds like a bacterial infection.
Hopefully, it will cure whatever it is, but there's no guarantee.
I highly recommend quarantining any wild-caught loaches for a minimum of eight weeks, and longer if possible.