Sump return

[krusty]

I have a J tube type overflow going to a wetdry unit which water trickes throught bioballs into a sump and then goes left through a foam block and the sits a a calm water area where the return pump is located.. The return pump, a rio 2500 is completely submerged yet at the return end at the tank there are hundreds of micro bubbles being spit out... what gives? no bubbles in sump, no air to pump, no leaks in tube.. yet there they are, stupid micro bubbles... any ideas, do you think i could rubberband a piece of foam around the intake grate of the rio... ?