A seeded sponge filter will help reduce the spike in ammonia so it shouldn't cause any deaths (hopefully). Just be sure you keep a decent amount of SW
prepared at all times for water changes. If available, get an ammonia alert badge made by Seachem. Cupramine being an amine based product will produce a false NH3
reading with most regular test kits. Be sure before doing a water change, the new water must be medicated before it goes in the tank to keep the Cupramine concentration correct.
The light on the QT
is not necessary and keeping just ambient light will help reduce stress in the fish. Although it does help if you have something to light the tank even if only for a few moments at a time. Nothing bright, just so you can see if there are any parasites present on the fish, secondary infection and monitor overall progress.