micro bubbles

[markus vaughn]

i have a 140 gallon tank i just set up with a 40 gallon sump i built.everything is running good except i cannot get rid of miro bubbles no matter what i try .i dont have a skimmer and i am using micro filter pad but dosnt seem to help.i have a 3100gphpump i bought off ebay.it is a pond pump but is rated to be used in marine water too .i just cant figure out why i have bubbles please help

 

[ToXIc]

how big is the return section?
do you have a bubble trap in the sump?