QTing fish is highly recommended but more work if you don't have everything in place such as a "seeded" filter. You mentioned your stepdad drips then tosses in the DT and has never lost a fish. This maybe because the DT is mature and established, unlike your QT so less chance for stress and low O2, high ammonia, or just parameters that can change quickly in a QT. sounds like something was rushed and the death could of possibly* been avoided. You may want to look into getting fish from somewhere that offers a 14 day guarantee until you figure out why you keep loosing fish so quickly. In the future one thing that will help with stresses leaving the light off for a day or two when putting fish into a new tank, whether that be your QT or DT. When I QT fish they don't have a light on the tank the entire time I QT them, I only turn on a light every few days to check the fish for signs of disease