It appears that bypassing my water storage tank has helped in eliminating my ranchu parasite problem. I'm almost certain that this (and potentially, buckets and hoses used in conjunction) had been providing an ongoing parasite supply to the ponds that were being treated. I overlooked this, as the tank had two permanganate treatments and shortly after appeared clean. However, on a recent refill, parasites were found when the supposedly clean water reached temperatures above 20 degreesC. Whether the Permanganate was not effective as a disinfectant or contamination was the source, i'm not sure.
It has shed some light on the apparent lack of effectiveness of any previous treatments. For example, it may have meant that during the FMG treatment, fresh parasites were being unintentionally added to ponds with each water change and not enduring the full course. The same may also have occurred with the salt treatments, giving the impression that they were resistant to the salt.
I'm hoping that any parasites on the ranchu themselves, would have gone through the full courses. The ranchu continue to appear to be in great health, showing no signs of infestation and reassuringly, scrapes of all the ranchu, have showed no parasites today. I look forward to getting the ranchu back into their main pond when i can begin preparing them for a now slightly delayed, winter period.