closed systems (our tanks) are very easy to pollute. think about every piece of food we add to it. that doesn't come out until we take it out.
along with keeping the feeding to a minimum, you should also be using every tool at your disposal to remove what you put in.
water changes and a good quality protein skimmer are the 2 main things. phosphate and nitrate removal media is also an option.
this is another reason folks should be careful how they stock their tanks. overstocked, an aquarium stops being enjoyable, and starts being a relentless uphill battle to keep from killing everything/ the tank looking ugly.