1) Did you rinse the filter bag in hot water before use as the note in the filter bag advised? If not the bag introduced phenols which will cause this reaction until they break down or are removed by carbon, waterchanges and skimming.
2) Have you added any type of surfactant additive or other filter media which may contain phenol? Any sort of stress coat, dechlor, slime algae killer, vitamin/amino supplement or new filter floss can cause this, so can some resin filter media.
3) Have you adjusted the air at all, it will need to be partially closed during the start up and likely even closed a little long term but less than when new.
4) Is the filter bag holder adjusted to the lowest point on the pipe and no more than 7" of the skimmer submerged. Ideally 5.5-7" of the skimmer is submerged, deeper submersion is possible but more likely to result in an overflow.
5) The only technical cause I can think of is a wrong needle wheel, last year we had some units in the US that shipped to us with the 50Hz impeller which has 2.5 circles of needles, the 3rd outer most row is cut to half height. The 60Hz version has only 2 rows.
