I started the QT process after already having fish in my DT. Have not seen any problems with any of the fish but it's not to say there isn't any. Fish looks healthy but that could be deceiving. I want to add new fish but I am concern with adding fish that has been thru the QT process become ill after being put in the DT. Is there a way to test the DT and it's fish inhabitants to see there is any hidden illnesses before adding fish from the QT process? Like I mention earlier I'm starting the QT after having fish in the DT already and the fish in the DT seems healthy.