Sorry to steer the topic in the wrong dirrection, but ScubaMedic, when you clean/change your filter, and fill it back up, that is probably when you getting your air in, after its running, tip it to the side a little, and it should blow out all the air after a couple times, instead of having air coming out here and there. Also, I think some bubbles are probably getting caught in the prefilter sponge when you fill it. I have a 404 and a 304 on my FW tanks and I have the same problem....cs