What pops said...
Based on the amount of filtration you have, and your amount of water changes my guess is your tank is overstocked. It may not be so overstocked that you must get rid of some fish or get another tank, but you will need to up your water change schedule (as others mentioned).
One thing is be careful doing huge water changes. If the new water is not oxygenated you will suffocate your fish. This happened to me when I first started out because I was doing a huge water change after my nitrates creeped up. Ended up losing almost an entire 55g tank's worth of african cichlids - needless to say a bad day that I still remember.