Couple questions.
Turn your Visi-Jet all the way to the + sign.
Are you getting bubbles in your pump? You can see through the little window. If not there is a little red plug on the top that you need to yank out and install the little rubber hose with the little twist lock air cap. Assuming you are not getting bubbles and have the tube already installed unscrew the cap and let some air in.
Are you getting tiny air bubbles throughout your downdraft tube, possibly spilling into your tank? If not see above, if so there are a couple simple modifications you can make to the visi-jet to make it operate better.
Purchase some filter floss and wad it into a ball. Place this ball of floss into the bottom of your downdraft tube. You want enough so that the bubbles stay in the tube, rising to the top and thus moving the gunk with it, but you do not want so much as to block the flow of water. You can check the clear section below the collection cup to see if there is too much floss fairly easily. Mark the water line before inserting floss with a pen. Add floss. If the water level moves upward take some out until you stop bht bubbles, and the water does not rise.
Secondly you can purchase a small air pump for about 10 dollars at your lfs. Buy about 3 feet of tubing and a limewood airstone (most lfs stock limewood airstones for "Lee's Skimmers" these will work fine for our application. Attatch the airstone to the end of the tubing and stick it in the bottom of the downdraft tube. Pack the floss around it as mentioned above. connect the other end to the pump. This will increase the bubble production. Even though the Visi-Jet is a venturi system, it is necessary to turn it down to about 50 gph to keep it from sending huge ammounts of bubbles into the tank. taking these steps will give you more surface area without having to sacrifice your tank to the bubblemonster that the Visi-Jet can be.
As a last alternative, if you have a tall tank you can replace the downdraft tube for a longer one... this can be hard as it isn;t always easy to find the same inside diameter tube, but it can be done. This will allow you to both turn up your air mix, as well as your gph on the powerhead.
Hope that helps.