[Indyws6]

Greetings...
A couple of things...

First, if you only have "clean" saltwater with a few pieces of base rock in a tub, there really won't be enough disolved organic compounds for the skimmer to act upon.

Second, even if you put the skimmer in action on a running tank, 8 hours is not enough time to expect the bubbles to flow over into the collection cup. For reference, after a thorough cleaning, my Reef Devil Deluxe can take as long as 24 hours before it produces enough foam to start overflowing into the collection cup.

You might want to consider putting more bio-balls in the tower. Using fewer balls creates more turbulence (and more foam), but it also makes the skimmer "twitchy" and more sensitive to tank changes. I tried using fewer balls to increase foam production and found that there were wild swings between no foam production and overflowing the cup. I eventually settled on using 9 balls in the tower, which seems to be the "sweet spot" for my tank.

P.S. If you haven't already done so, get a piece of fishing line and tie it to the first bio-ball that goes into the tower, leaving some hanging out. You can use it to pull all of the balls out for cleaning, or if you decide to change the number. Beats disconnecting the skimmer so you can dump them out...

Hope this helps...