For some time now I've had an erratic skimmer function that I've tried to figure out but to no avail. TMC reef skim 300. So, as a quick overview I have a sump with a level marker in the final chamber housing my skimmer, return pump and auto top up. All TMC, the auto top up does it's job well and is very sensitive so the level stays constant. The problem is that after a water change I have to dial the skimmer right down to get a dry skim and then over the week adjust it up gradually. It's very hit and miss, it can be skimming fine, slowly when I go out to work, then overflowing 10hrs later for no obvious reason. I've thought about varying O2 levels but considering some weeks it can go 5 days before playing up I'm not sure. I'm working on a new setup which will have a new 1000ltr DC skimmer and would like to get it right before the changeover Thanks.