Questions - Need some input

Hello everyone I am new to this site. I just wanted to say hello and introduce myself. My name is Gina and I have a 183 gal cube tank. I recently had a battle with ich getting into my tank. Still haven't figured out how but in any event it killed most of my fish. When I first started seeing signs I pulled all the fish from the DT and put them into a 75 gal QT tank. During the ich TX most of them died. Not sure what went wrong. After about 3 weeks in the QT just one by one the started to go. Eating ...