I have a etss pro 800 the problem was to dry of skimmate.If I closed down my output valve I noticed a drop in air coming in and it became hard to control the bubbel collum, so I brought 1 12 inch acrilic tube the the same id as my down draft tube and cut it in half. Then I brought atube the same od as the id of my first tube cut 4 1n long and glued it half way in the first tube. I now have2 tubes that slide in my my down draft tubes and I added some bio balls . This took care of my problem and made adjusting less touchy. I now have to close my valve on the input to adjust my skimmer. This works for me just a thought.