All I’m saying is that the level you are pointing out is the normal interface. This interface is harder to see when the skimmer is fractionating out more organics (working better), and easier to see when the bubbles are weak and pop more quickly. So I think your issue is that your bubbles are not being allowed to form up correctly due to something, from time to time. I notice when I add certain things like some chemicals or some certain foods maybe that my skimmer makes weak bubbles and fractionates out less sometimes. I didn’t dig deeper because it doesn’t seem to cause other problems in my case, so I don’t have specific suggestion of what to look at for a cause, other than just that it might be something you are adding as opposed to something wrong with the skimmer.I hear what you are saying.
However, I’ve been watching this for two months at this point. I have two skimmers, both Red Sea. The Frag tank and display tank are ten feet apart. This bubble interface happens and the skimmer won’t produce skimmate until I take the skimmer out and clean all the parts (the skimmer has no buildup on them). Which has become as frequent as every 4-5 days, and that is why it’s a struggle. Meanwhile the frag tank skimmer hums along without this issue.
So that indicator for this skimmer is abnormal and impacts performance.
When the skimmer is operating correctly the entire body down to the top of the pump is the milky white color and collects skimmate.