No one uses TTM or chloroquine phosphate anymore? To the OP, it’s ich. Your established tank mates have had it but may not show symptoms. Treating the Achilles will do little for him if you plan on reintroducing him to your DT, since he will be reinfected. Only way to rid your DT of ich is to remove and treat all fish and leave it fish less for 72-76 days. Depending on tank temp. Some say higher temps of 82+ it’s 45 days. I have only ever done 76 days at 77 degrees. If it’s a FOWLR, you could dose CP providing you don’t have wrasse, hippo tangs too. There is a chart somewhere on here