constantly?
try using a little less powder. my xl seems to "burp" and blow a blast of bubbles now and then as it gets restricted. another way to tell if it's restricted is by looking at the bag. is it like sucked in?
i *think* what's happening is the impellor actually churns the water so hard it creates the bubbles, or it's just filtering them out of the tank and the blast happens when enuf collect.
it might help keeping the filter lower too. i usually set mine on the floor in front of the tank. watch where you position the inlet too. if your in a regular filters outlet stream that could be where the extra air is coming from.....
and... as nj devils fan already said..... look for leaky connections on it. maybe one of the inlet connections is pulling a little air in.
i just ignore it, but mine is only occasionally, not constant.