Why does my return occasionally belch out microbbles?

There are no microbubbles in my sump.

Is my pump cavitating or something?

 

Maybe or there is a leak that is pulling in air somewhere.

 

mine only does that when the sump water level is too low,pump creates a vortex and draws air in

 

You probably have a leak.

Where is the anti siphon hole in the return line?

 

Exactly where my thoughts went. Find the anti siphon hole on the return and put your finger over it and see if it resolves the issue then update us -- many times they get encrusted with salt creep and suck air.

 

Maybe your fish ate too many beans

 

A vortex caused by cavitation is often responsible when bubbles are intermittent

 

I had that happen because the "water level line" in my sump was lower than the height of my return pump. I had to add more water to the system to get the water level a good bit above the pump.

 

If you have a "typical" sump, look under the large sponge that is just before the return pump and see if large bubble are under the sponge. From time to time that happens (sponge needs cleaning) and the large bubbles can work their way to the return pump where they are promptly blown into display as almost micro bubbles.

 

I second the fish eating beans theory.