I'm not Dmorty but I'll tell you some of the things I observed.
I decided to treat the display tank, too, because the fish was too hard to catch. I had fish, two kinds of cleaner shrimp, hermits and snails. There were too many snails to remove, all down in the sand and behind rocks so I counted them as a loss and depended on the hermits to remove them when they die. I have sand. I dosed as per the directions.
At first the snails just came out of the sand and even out of the water. I didn't have anywhere to put them so I left them in there. They died by the third day. The hermits and cleaner shrimp acted normally and continued to come to food. The fish cleared up. Right before the 3 weeks was up, the cleaner shrimp started staying in the same place and waving around. They didn't come after food (pellets). The hermits were unaffected. I reintroduced GFO, GAC, and protein skimming. I did a 1/3 water change one day and another 1/3 two days later. Two of the cleaner shrimp died, three got somewhat better and one remained dazed.
After a little more than a week, the fish got spots again. Dmorty told me the treatment doesn't necessarily cleanse the tank since the powder doesn't kill all life stages of the parasite. After giving the fish a few days of trying to kick the ich, I decided to try again. I know, lazy right? I figured that maybe the life stages would all cycle and I would get them this time. I removed the dazed cleaner shrimp but the three that got somewhat better went into deep hiding. I dosed twenty percent higher than the original dose and decided four weeks instead of three might be better. The safety of this was approved by the directions.
The three shrimp died almost immediately. The one dazed one I removed recovered completely in another tank. The fish cleared up immediately. I went five weeks and didn't do water changes but did start up carbon, skimming and GFO. After a little more than a week the spots came back on the fish.
The powder kills the algae very soon after you put it in. You know it's gone when the algae comes back, which it does soon after the four week mark if you skim and carbon filter.
That's why Dmorty and others recommend treating in a hospital tank. The stuff definitely works on fish but it doesn't cure the tank. At least, that's what I've found.