Yup, it's hard to administer octozin effectively in my opinion, it didn't kill off all the ich. I had a bad outbreak 1 year back that took away 1/2 of my fishes when I added too many fishes at 1 go thinking my tank is matured enough. Tried all sorts of methods but fishes still died, some I suspect due to stress from treatment too!!
Now, I have about 30 fishes in my tank but ich is no longer of a concern (fingers crossed) as they are resistant to ich. Whenever I add a new fish it will bound to have ich even after quarantine when release to my display but it doesn't affected the rest, even my 3 Tangs, unlike a year ago which the ich will spread.
What I did was to improve my water quality & feed daily with soak food (garlic & vitamin) after the outbreak a year back.
I have boosted the bacteria variants by dosing different types of bacteria (bactiv8 NPX, biodigest, a special power form of bacteria bot from Henry) plus fritz zyme to remove waste materials. This make sure all the waste, nitrates, phosphate are remove. Also using biopellets to provide carbon food source for the bacteria to grow.
In short, stop adding fishes & let your bacteria populate the tank & add different strains of bacteria as they have different functions. Overtime, a healthy bacteria population will provide better environment for yr fishes & less likely they will fall sick.
Lastly, regular water change & have a good skimmer to prevent bacteria boom if overdose (signs : cloudy water but it is fine).