Micro Bubble everywhere

[Jason McK]

So I Fired up my 40G with a DIY overflow and closed loop. The Closed loop is spewing Micro bubble like mad. I turn off the CL and the bubble were gone.
Now how can I determine were the air is coming from
The input is PVC Pipe running from the bottom of the tank over the side and then converted to Flex and into a MAG 5. The output of the MAG 5 is flex to a SCWD and the outputs of the SCWD are flex then PVC and hard plumbed to bulkheads in the side of the tank.
I have no water leaks even with power off. and the bubbles are coming out of both returns.
So I would assume the air is getting in before the SCWD.
Is there a simple test to find air leaks

Thanks
Jason