The thing with ich is that it cannot be killed while it is embedded in the fish, or as an egg. It can only be killed as a free swimming tomite, on its way to a new host. So for using formalin type treatments, they need to be done almost daily for a couple of weeks, depending on temperature. I prefer salt at 0.3% or higher, up to 0.6% until the fish are symptom free of the parasites and then the salt can be removed during water changes. Salt will kill the tomites, and it is always there until diluted out of the pond, so it is more effective than the other trreatments.
Zone 7 A/B
Keep your words sweet. You never know when you may have to eat them.
Richard