okay thanks for the extra information. I don't feel safe taking all the fish out of the DT due to the yellow dying in quarantine. I think what ill do in future is cycle a marine pure cube and put it in with the fish for some extra filtration each time I quarantine. As for future fish I think ill put them in the quarantine tank, observe them for a little and treat if needed.Again. Sorry for what you and fish are going thru. I know how difficult it is and I hated it each time.
Like me it's a lesson we are learn, and sometimes like me takes a 2nd lesson to really stick to do proper QT next time.
That red spot on your yellow Tang, I've had that happened to a fish after disease and death.
It's most likely not the heater (since water dissipates heat very quickly).
It's most likely internal organ damage that show ups for whatever reason (disease or treatment).
As far as Fallow Period to restore your DT to safety....Read this.
https://www.reef2reef.com/threads/fallow-periods-going-fishless.190324/