If you are using some chemical to somehow try to kill off the hair algae or whatever, the LFS may have meant you should turn off the carbon filter for a certain period of time to allow the chemical to do it's job. If you don't, the carbon would probably absorb most of it. With some chemical treatments they advise to turn off carbon and even skimmers for a certain time period and then running heavy carbon to clean up the chemical after it's had a chance to do it's job.