If you can safely rely on the check valve, trying hooking up some flexible tubing from the pump to the tank instead and bypass your regular return line and see if the bubbles go away. It is possible that air can be sucked in creating the bubbles without water coming out of it.
Also, try raising the water level in your sump a bit. I had some microbubbles in my 450g that went away once I raised the sump level a few inches. In fact, when I start seeing microbubbles I know I need some top off water in the sump.
Other possibility is I see you have bioballs in your overflow. Presumably that's too quiet the long fall of water into the overflow. Still though, that long drop can cause increased bubbles as well. If so, try raising the water level in the overflow with some PVC.
OR, if you are using a submerged inlet in the sump from the overflow, that can help to quiet flow, but greatly increases microbubbles. Additional sponges and/or a bubble trap can be used to eliminate the bubbles from getting to the pump.
Or you can use a right angle adaptor on your pump so you can suck water from the bottom inch or so of the sump so that you don't suck if hopefully they've risen towards the top enough where they won't be sucked in by then.