What do you have for flow? I'm sort of thinking that you don't have enough flow if all you're running is a HOB filter (specially if it contains any media) for a 30 gallon tank. I run a filter for 20 - 40 gallons (no media, mostly a place to put the heater so I don't have to look at it
) plus a further 100+ gph powerhead in my 10 gallon, and I almost feel like I want more flow. I estimate that I have ~300 gph running around in my tank.