Unfortunately, more than likely, the ich is already in your tank. I just went through a major battle with ich in my 55gal mixed reef. Casualties are... 3 pajama cardinals, 2 dottybacks, yellow tang, bicolor blenny, and my manderin goby. All deaths acured within 3 days. With only one pajama cardinal left, i was pulling my hair out on what to do. I spent hours researching, and finally made the decision to do a quarrentine tank and start hypo salinity treatment on the cardinal. I went to walmart and spent a whole $35 on a 10gal complete tank setup and a $10 heater. Took 10 gal from my DT and poured it into my 10 And in went the fish. Within 1 day he was showing signs of ich with his eyes whiting over to the point of him looking like he had cadaracs. I slowly did 1 gal water changes replacing the tank water with RO water (no salt) till SG was at 1.010. (Over a 48 hr period) after him being in there for 4 days, he looks 100% better. Clear eyes, back to eating. I did dose the 10gal once a day with kordon rid fungus 1 tsp just incase he had any other infections. This process takes 8 weeks to complete. Leaving your DT fish less for that time will starve the ich parasite and it will die. I have a great article that I will post that really changed my mind on not getting a quarrentine tank.
Reefing is like a box of chocolates, you never know whatcha gonna get!