So afraid this might happen after hearing the health issues. Dinos can be very toxic depending on species. It’s the same thing as red tides in the ocean. I lost all but one fish and two corals after my Dino outbreak. Some died weeks later. Once the damage is done the fish just can’t recover, it's not a treatment thing, the toxins hit internal organs. It’s nothing you did or didn’t do short of having another tank to put fish in as soon as you realize it’s Dinos and not many of us have that (I don’t) and Dinos can be hard to recognize before it’s too late. You can look at my build thread for my Dino issues and the recovery of the tank. To be honest, the post Dino issues with GHA were as frustrating if not as lethal that in hindsight should’ve led me to rebooting the tank.
I feel for you, Dino are about the worst thing that can happen. Partly because once the fish/corals start dying you just feel helpless. It was as close to leaving the hobby as I've been. However, a year and half later (I think) with my UV running I have a nice looking tank, looking to go a bit bigger and really enjoying myself. But man, did I hate it there for a while.
I'm not sure I'd worry too much about seeding bacteria (or at all). That's not your issue in my opinion and I seriously doubt it's what killed your fish.