Evidence of Brook usually starts near the head as the parasites attack the gills. Have you noticed any labored, quick breathing or gasping for air at the surface?
We used to use Formalin to treat Brook, but that stuff is a nasty known carcinogen that can shorten the fish's life span. Gloves, googles, and extreme care when using.
A better option is to give the fish a bath in Ruby Reef Rally. The acriflavine in RRR should take care of the Brook in a very gentle way.
Other options are to treat with Metroplex.