I've got the exact skimmer...sitting in 28cm water level and dial set to between the 1 and 2...this leaves the bubbles just above the red part of the cup... If it does ever overflow it's either because of a higher water level, clogged airfeed into the white plastic venturi fitting on the pump, clogged outlet or directly after putting in an additive like phytoplankton....... It could still be running in, but you can check the air feed and the outlet tube for any blockages