I agree, I've tried them before also with a Hippo and Purple tang in my tank and no luck, they were ripped right outa my substrate and chomped on
I guess lighting and timing have to do with shaving brushes, I had a friend who did well with them and had only one yellow tang grazing in his tank. He also had some great lighting on his tank and had the shaving brushes set up before he got his yellow tang. I guess they rooted well and the lighting kept them growing quiker then the tang could eat...
What do you have in your tank? Could you update us later on to how they do.