Todd's been doing this for about twice as long as me. I've never had anything that severe happen. In a bit over 10 years I lost 1 fish to ich. It was an achilles tang. Good luck I guess.
Meanwhile, you could use the 20 as a qt. You don't even need a "filter". You could just leave some media in your sump of your DT, like some ceramic rings or bio balls for a month or two and toss them in the QT tank, or you could try that bottled bacteria. A power head or two and some PVC
pipes/elbows for them to hide in, and there's your QT.