Yeah I seconded that idea...as I have tried different methods like sock, coconut bag, tap filter bag, cloth mesh pouch, extra pipings, cover outlet mouth with filter wool, etc....only to discover that they solve the micro-bubbles problem for the moment.
...until I tried this cost-effective method that is to insert a big rectangular piece of stiff black filter sponge (which u can buy from any aquarium shop) in between the return pump & baffle at the last section of the sump.
The 'holes' in the sponge effectively trapped any big & small bubbles & of course dirt particles that flow through it.
Maintenance is easy as I just take the sponge out, rinse it, tap it to remove water droplets & then re-use... save money$ !!!
Mirco-bubbles no more...except now I have to deal with my resident 'bomber' in my main tank that periodically spurts out sands, mess up my corals & water clarity...
Cheers...