[Delphinus]

Well, mysis is fairly oily and shuts down my skimmers for a while. I assume the lack of foam production puts less back pressure on the pump feeding the skimmer, thus the water level rises in the skimmer, and correspondingly, comes out of the tank. When the foam production begins again, it puts a little back pressure back on the pump, the water level goes back to normal and thus the level restores in the tank (notwithstanding any losses to evaporation, or initial foam spillover into the collection cup, or water put back into the tank during the slightly low period by a float valve or float switch and so on).