If it is ich you need to wait i believe its 7 weeks for it to die out of the system, and this is only if there are no more host in the water, meaning there cant be 1 single fish in the tank for 7 weeks.
The only thing that sucks about this is that you then have to treat each fish you get with hypo salinity or cooper in the QT tank for atleast 3 weeks Some do longer Just in case. And then add it to your DT.
i had a ich outbreak when i first started my tank, And i know that ich still lives in my system since my firefish never died and are still in the tank, i just keep all my Water in check and keep my fish happy and i havent had an out break for the longest XX's fingers. Everything in my tank is doing good, i do how ever run a UV steralizer in the DT with slow flow, witch i have read helps kill the bad stuff in the water and parasites.
So yeah if all your fish do die Your going to have to wait & weeks before adding a fish to be sure the ICH or parasite is gone with out a host.
then you have to Quarentien each fish for atleast 3 weeks and treat them before adding to the Display.