Barb, if this is a protein skimmer, and you have no livestock in your tank yet (as other posts suggest), there really can't be a reason to run the skimmer just yet. I guess, perhaps, just to know that it runs. But, until you have some organic waste in the tank, there is nothing for the skimmer to skim. As long as you understand that, and you're not looking for anything to collect in the cup. I only know this because I was frustrated trying to get my skimmer to work in a brand new tank just a couple short years ago...