Did you quarantine the original fish in your DT? if not there's still chance you have some nasties in your main tank and it hasn't had the opportunity of show itself. I had ich pop up years ago when the canister filter hose popped off in one of my tanks while I was in jersey and when I got home it had 4 inches of water well once I got everything fixed and refilled about a week later most of my fish had spots and I hadn't added a fish in over a year. so you might want to treat all your fish just to be sure