I would say no skimming for 24-48 hours but that is just my best educated guess. I would assume no skimming for that time period would allow the medication to have enough time to do its job. I dont know the chemical make up of prazi-pro, so I cant give you an exact answer. Hopefully someone who has used this product in the past can chime in and give you more information. I have used prazi-pro before, but was not running a skimmer at the time. I would think that the only reason you would stop skimming and running carbon would be to let the medicine work and not pull it out via mechanical filtration. Hope this helps