Not sure if you fixed your problem but wanted to share this, I have had a problem with micro bubbles in DT but it seemed I was only getting a burst of bubbles every So often, so I thought that I must be sucking air through a pvc fitting somewhere and that it was building up then eventually releasing all at once, but I have been home a lot recently and watching the tank more often so I happened to notice what the real cause was I was looking in the return section of my sump when the ATO came on I then noticed that it was directed right at my return line intake the top off was forcing air bubbles in the intake when it came on I simply moved it over and problem solved not saying this is everyone’s case but definitely something to look for if you are getting pockets of air that only release randomly