Mark - I believe doing it without a skimmer is fine as long as you do run the carbon and do water changes afterwards. Many medications/treatments instruct to turn skimmers off during treatment. Get as many of the flatworms out of the tank as possible before treating it. This will help reduce the amount of toxins released when the ones in the tank die. I've not used the flatworm exit product, but follow the instructions carefully. It doesn't take long for the flatworms to die so you need to be ready with new saltwater and the carbon before you start.