Why does my return occasionally belch out microbbles?

Yep, also too much of a waterfall by the pumps inlet will do it too. I solved a buddies micro bubble issue by having him place an acyrlic diverter to drop more water away from the pump inlet. I had this issue too when I had a broken tooth on my final overflow grate drop a tiny bubble into the inlet of the pump from time to time. I only had to turn my last grate around to the broken off tooth was not over the intake of the pump.

 

Thanks everybody.

None of the possibilities mentioned pan out except cavitation. That pump is not getting any external air, period, and there is no anti-siphon. The bubbles come in intermittent bursts. Maybe I'll take apart the return and look at the impeller.

I don't suppose loc-line can create a venturi effect can it? I get salt deposits on the exposed loc-line, which means it doesn't seal absolutely perfectly.

 

Is any of the loc line above the water?

That stuff can and will leak air.