Manual -
http://www.f3images.com/IMD/UserManuals/RS1135.pdf
For aquariums up to 1200l
IMO it can only support max 120l(3x1.5x1.5 feet) with mid bioload
How to tune your Red Sea Prizm Pro skimmer
start the pump and after the water starts to flow nicely
fully release the air plug(counter clockwise) [1-3sec]
and gently tighten it until the skimmer become quiet [4-8sec]
http://www.youtube.com/watch?v=AXsR24jeHHk
cheers