The problem, as stated, is your maintenance schedule. Once a week water change is pretty much the minimum. I do 2x a week on one of tanks, since it is stocked a bit on the heavy side.
I would recommend you do a once a week 30-50% water change, and clean the filter every three weeks (make sure you rinse the bio media in tank water, not tap water). I bet you notice the difference immediately. Your tank will stay clean and water clear.
Also, bigger filter is always better, but a good maintenance schedule is more important IMO.