I had success treating a group of chromis with Uromena with chloroquine in a QT. They cleared and the sores resolved. It returned when they were placed back into the DT and I lost them one by one over a 2 month period. I'm not sure it will completely eradicate the parasite in a DT since it is not an obligate fish feeder and the mechanism of action in chloroquine may require the fish. In a QT, the bottom is bare and feeding probably relies on the fish.
It is worth a try because treating a DT with formalin isn't a great idea. If you are having trouble finding chloroquine, PM me.