Just cleaned my 6gal nano. I had some hair algae on the water output ( if that's what it's called? ) then it started to produce a crazy amount of tiny bubbles. so much I can barely see anything in the tank.
I wasn't keeping the reservoir water level up high enough in mine, so when water flowed over the sides into the return pump sections it produced all those micro bubbles that got sucked up into the return pump and spit back out into the display tank.