Sure i can back this up...
I stock my tank on day 1 using decent rock - to wait 5 months is mad.
To wait 5 months to cycle the tank is a collosal waste of time and space...
Even to mature a tank in 5 months is a load of huey.
I started my tank in about May and stocked my tank with no LR or LS after 1 day with damsels and clownfishes, and even a cleaner wrasse, scooter blenny, and a feather duster within 3 weeks, the green chromis and the feather duster are still with me and the rest kicked the bucket of introduced diseases. Hell, I even had a mandarin dragonet for a week, back when the tank was two months old. he appeared to "shed his skin mucous" then croak few days later. This was probably stress induced, he was being pestered by a hawkfish. I checked him after death, he appeared to have been eating well. But anyways, did this happen? Everything else I could have suggested has already been.
But sometimes, fish just die, it may have nothing to do with their caretaker. On time, I had about six angel cichlids, and one died of infection, and the rest, (one was a monster) just sort of followed it to hell. To this day I cannot explain why they died, this has happened many times. maybe it just happened to you this time.