Hi Marco,
I have used 3 different models of his skimmers. Perhaps I can help. What height model is it? What pump are you driving it with?
I found on the taller models, 36in and 32in. I had to run the water level just under the cup, to get better production. IMO, these large skimmers could be run lower, using a larger pump, like an Iwaki,70 or something. If powered by a Mag 24 or a Mak 4, which are the minimum pumps, then they should run a higher water level. It also depends on what its skimming and the tank load.
Jayson runs the 36in. models on his 500g tanks, powered by Mak 4 pumps and they skim like crazy. He runs the level at the flange. But his load is much different than ours.
I now have one of his newer sized models. Its 27in. and when driven by a Mak 4, it rocks. I have been making tank changes, so just played with the skimmer so far and it was producing like crazy the first night. I run the water level in it, just above the flange.
FWIW, I have never seen better quality in any aquarium product I have purchased.
