The only time my FX5 had micro bubbles, a sponge had covered all of the holes in the middle of one of the rings and water flow was struggling to get through.
As for the 404 popping open, like others have suggested, I do not see how this was possible. My only thought was that the gasket went bad and water leaked from the head of the unit as a result of back siphoning when the power went out.
I run fluvals on most of my tanks (4- FX5, 4- 404, 1- 204). The only problem I had with water leaking was a bad gasket on a 405 a couple weeks ago. Even with that, the water trickled out and down the canister sides.