Guess I was too optimistic about how many fishes the aquarium could house. I have measured nitrate and phosphate a couple of times the last months and they were at a steady high despite using rowaphos and reef pellets for removing nitrate. I have added a reactor chamber run by a small eheim pump (around 400 l/h), but it did not help much, nitrates where still around 2-4 mg/l and phosphate around 0,4 mg/l.
So last weekend I changed 600 liters of water and cleaned the sump and the upper filter aquarium. It also seems that the upper filter aquarium did acts as a filter as I hoped, so I removed some of the live rocks and placed them in the sump. Now the upper aquariums acts as a crustacean zoo, with at least 4 species of crustaceans, to keep them from eating my snails and being eaten by the turbot.
The turbot by the way ate the small lumpsucker after just one week. After the water change the nitrate and phosphate is much lower, the big fishes are gone, so I hope that will help.
Here is a photo of the upper filter aquarium and one of the squatlobsters.