The pressure release button is for when you have to open the housing for filter changes during normal household use. Mine does not have one. I'd imagine you could use it to burp the air out as your canister fills it. The longer the housing, the less chance you'll have bubbles. Yes I would fill it with something like sponge. I put a big old sponge filter in mine. Anything you add will also double as increased biological filtration anyhow, but may further reduce flow. As for micro bubbles, I couldn't tell you yet as my substrate is still releasing air, this is a new tank.