Are you hanging it on the back of the tank? This is what I did
Have had it for 1 week)
1) It does need to build up a little bit of slime, give it a day or two.
2) Open the valve all the way and then screw it back a quarter to half turn (barely catches). Although this goes against the direction of the book but most people do it this way. Look at the tube coming from the powerhead going into the base of the unit. It should like a whitish foam almost.
3) Make sure the powerhead and sponge is 3-4" under water
4) Make sure that the return lip is slightly under water
5) If you are really concerned about the amount of bubbles going into the water you can put a fine fish net onto the return lip as this will break up the bubbles.