Har hittat lite bra info som jag vill dela med mig. The air-intake must be positioned on the exit side of the restrictor just after the narrowest section of the restrictor, not in the centre of it. VERY IMPORTANT: The air-intake should be quite narrow (about 3mm) in comparison to the water inlet (22mm) and the bore of the restrictor (approx 10mm), so that the vacuum on the exit side of the restrictor can act easily to pull air in. This is the main mistake people make when constructing a venturi. The larger the diameter of the air-intake pipe, the faster must be the flow of water through the venturi, especially if you want it to pump into deep water. The air-intake must be cut at a slight angle (about 20 degrees) to match the cone shape. The more narrow the bore of the restrictor is (start with about 8-10mm), the faster will be the flow of water through the restrictor resulting in a more powerful vacuum to suck air down the air-intake pipe, but the trade-off will be less water pressure out of the venturi into the bio-filter (this isn't a bad thing since we want a controlled flow through the bio-filter to give it enough Retention Time for bacteria to work), but this might also mean any water feature following on would not look very effective and it could put a strain on the water pump. /Jens