Hi Arjan, I'v had tried many way to modidy the skimmer and the result was the same just filling up with tank water. So disgard it and got a Jad wg-308 skimmer with it own pump and it fit well inside and does the job better than the original. You can get it on Ebay now as it was hard to get.HTH
So I tested the pump without the UV part and without the skimmer part. The pump had a powerfull current so this was not the problem.
I made a new PVC inlet with airtube for the pump and tested it again (without UV). Now the pump gave a powerfull current with lots of bubbles. Then I took away the PVC part and put on the UV light. Again lots of bubbles. So I put the pump with UV light back to the skimmer part. Almost no bubbles ?????????????
I took out the skimmer tube and looked at it carefully. In the inlet (where the pump is connected) I saw a verry small hole. It was a ring. I took the ring out and connected everything back together.
You can see the result after a few hours on the pictures.