Ive never tested for nitrates or nitrites so I have no idea what a store bought filter would have compared to my filter. All of my fish are wildcaught and they are from less than pristine waters if you know what I mean. The water quality in my tank far exceeds that from which they came, so far as I can tell. Maybe ill have to get one of those kits and test it.
All of my fish are hardy species which can survive the pollution given to them in an urban creek system, which flows into the Kansas River which has health warnings that fisherman should only eat 5 oz of fish caught there per month.
The only fish Ive lost to other than predation was one whoose habitat requirements and food supply was not met, a central stoneroller, which I now understand is tough to keep alive in a community tank. I also had a couple of bass that had an air bladder problem, one recovered and the other was eaten before he could recover.