I agree, Cp would certainly be my first choice of treatment. I can't see any reason for using both treatments.
The reason for asking is that I have read of a couple of guys treating with copper and then after removing the copper from the system a few days afterwards treating with CP. In both cases the fish have reacted badly to the CP.
In my view there wasn't a long enough period between treatments? The fish will have already taken a knock with the copper before subjecting them to a further treatment. I am trying to ascertain whether there is any information on how cp may react with any residual copper left in the system?
I believe there could be a few reasons behind this.
Firstly, copper is a toxin, it's detrimental to both fish and parasite health and some fish tolerate it better than others. In any case, a recovery time needs to be implemented between courses of any medicine to clear both the tank and the fish's bodies of the medication, ensure they are feeding etc.
Secondly, diagnosis. Copper is broad, and it's often better to use a wide ranging treatment like copper if the diagnosis is uncertain. But isn't a wonder treatment. There have been reported strains of Cryptocaryon and Oodinium Occelatum which appear hypo, CP or copper resistant. It could be that even if correctly identified, a resistant strain is present.
Thirdly, ammonia - QT tanks typically have very immature filtration systems and ammonia build up and spikes are a big problem. These either directly affect the health of the fish and/or allow something else to take hold.
Lastly (and I refer to treating with CP here as that's where my experience lies), treatment can often lead to bacterial infections taking hold. I have experienced this a couple of times and copper and chloroquine are of no help in this case. In my last full tank QT, I was forced to abandon CP on day 20 and quickly introduce an antibacterial treatment due to this taking hold.
My preferred treatment is always CP, but there are times where another medication may be best applied