Brook usually kills very fast, most fish die in 3-5 days if not treated with formalin. There is this distinctive white sheen on the body, flared gills, swimming at 45 degrees. Velvet is also a fast killer. You will see fine white dust on the bottom of a container after fw dip, if infestation is high enough.
I would switch to tank transfer method in combination with copper, if your fish get re-infected with ich. Take close up high resolution pictures of the infected fish and post it here for ID.