well teh first thing that comes to mind is that you have to many fish fpr that system. even with the water changes its not enough, the system just cant handle that bio load. you might have to do 50-75% water changes to sustain low nitrates. for your set up most would recommend just keepin acouple fish, if having more they need to be small when fully grown. thats the only thing that can think why you are having this problem.