Hi bro,
Unless you willing to spend some $$ to change the skimmer if not try these method ensure your SG level at 1.022 - 1.024, take your whole skimmer out buy 6 bottle of white vinger and soak it for and hour. Then fire it up again and see the result, reason for doing that is u will see a thin flim of oil floatring on the water for some new skimmer...... rem to rinse is with water hor dun just take out and dun wash ur PH hit rock bottom not my problem huh.....
As for the micro bubble...erh.... sorry bro..... no cure.
This skimmer used for a nano tank is okay but u need to spend some time to tune it. This skimmer is consider a decent skimmer, alternative you can consider the reef octo skimmer, if u had deep pocket Tunze 9002.