MSX skimmer emitting microbubbles

[TonyB]

I have an MSX 200 skimmer. It just started releasing a lot of microbubbles from the output standpipe. I removed the pump, gave it a good cleaning and inspection, then replaced it and the problem persists. Does anyone know how to remedy this problem? I had it sitting in ~11" of water and it ran fine but I thought it may be a bit too deep so I moved it up into about 7" but it didn't help.