That 1 fish would more than likely only add a little Ammonia. I think your testing is off. I would test again, and maybe even get a second opinion. I clean my sand beds in my tanks so I don't hold nitrates or detrius. If you can, when you pour in new water, pour it into the filtering / sump/ holding area. That way it wont blast the sand bed or corals all over, and it will mix with the current water you have.