If your skimmer is new you want to wait 3-4 weeks before adjusting it to allow the skimmer to break in. During this time the skimmer will be very unpredictable and inconsistent. After the skimmer has broken in you want to try to retain pea size bubble and larger where the collection cup attaches to the body. Run the skimmer like this for a week or so. After this period if you choose to you can open or close the valve in small increments every other day till you retain the desired collection consistency. Never make changes to the skimmer after cleaning, feeding or handling the tank.