Maybe this will help:
The ich protozoa goes though the following life stages:
Feeding stage : The ich trophozoite (a protozoan in active stage of life) feeds in a nodule formed in the skin or gill epithelium.
After it feeds within the skin or gills, the trophozoite falls off and enters an encapsulated dividing stage (tomont). The tomont adheres to plants, nets, gravel or other ornamental objects in the aquarium.
The tomont divides up to 10 times by binary fission, producing infective theronts, thus dividing rapidly and attacking the fish.
This life cycle is highly dependent on water temperature, and the entire life cycle takes from approximately 7 days at 25 °C (77 °F) to 8 weeks at 6 °C (43 °F).
KEY POINT - NOT every infective theronts is born on the same day AND they are NOT all killed by the treatment one the same day. Hence the time honored 30day Quarantine period.
The reason I double dose Ich Attach on the 1st three days is to wipe out as may free swimming infective theronts and ones just hatching as fast as possible giving the fish a chance to start getting healthy enough to bounce back.
In a severe cases where a fish was heavily infected, I would initially do a salt bath for 5 to 10min. at 1.005 to 1.010 SG(specific gravity) based on the species. I prefer marine salt to use as a bath. Be advised a failing fish may not handle this well. At the fist sign of sever reaction get the fish out of the salt water bath. I do not Salt Bath a scaleless fish like loaches.