The "main reason for ich being able to do its thing" is that it's a parasite, and that's what parasites do. Yes, stressed fish are somewhat more vulnerable - there are those who would argue that a hippo tang and a porcupine puffer in a 55 gallon tank would already fall into that category.
So with the polyp lab "conditioner", you dose this stuff (peroxide) twice a day, just after the lights go out, and somewhat before they come on, in hopes that you'll make the ich parasite uncomfortable enough that the fish can stay ahead of it? And maybe it doesn't damage your corals or kill your shrimp. (Shrimp and peroxide are not a good combination... then again, shrimp and porcupine puffers generally aren't either . . . )
Or, you remove the fish from the tank, and you kill the parasites, and you keep the fish separate from the display long enough for the parasites in the display to starve to death, which they will, given time, as both ich and velvet require a fish host.
JacquelineVictor, you may be in a difficult spot if you want to completely eradicate ich - the two best chemical treatments for both ich and velvet are Chloroquine Phosphate (CP) and copper (either ionic copper such as Cupramine or chelated copper such as Coppersafe). Puffers don't tolerate copper well at all, and hippo tangs don't tolerate CP. You might look into TTM (Tank Transfer Method), but this will be challenging with as many fish as you've got.
Very likely that
@melypr1985 or
@Humblefish can offer deeper insight and better advice. There's nobody I know who I trust more than those two folks when it comes to getting fish through disease.
~Bruce