I used the Prizm on my old (55 gallon) tank successfully for around 3 years... initially I had similar questions and wrote to RedSea and received the following guidance (which worked for me):
When putting the Prizm skimmer online you will have an overreaction for the first 48 hours. After priming the unit and moving the black tubing to the # II hose barb, immediately move the flow regulator valve to the completely closed position. Run the unit that way for the first 48 hours so the unit can settle down on your water in your tank. After 48 hours, open the valve a small amount and you should start collecting some dry type foam in the collection cup. After the 48 hours period, try not to add (Water change, water additives, stress coat, calcium, etc.) to your tank for a couple of days, to insure the system is settled.
If you continue to have a problem with bubbles being pushed to the tank and the collection cup filling with water, try raising the skimmer up off of the tank anywhere from 1/2" to 2" to give the skimmer more reaction space between the skimmer and the water.
If your tanks is so new and not much load, you really do not have hardly any waste protein to burn off.
As for over/under skimming... The center section of the skimmer will be very turbulent. You will have the unit turned too high if the collection cup fills with water only and there are micro bubbles being pushed into the tank.
Hope this helps... let us know how it goes.