If I was to use goldfish I will be using the best quality show goldfish we can get in Australia. They will have lots of problems eating taddies. I will probably just use delicate blue eyes as I love that species. I use plants to filter the water of most of my fish tanks, the amount of waste they produce is realy not much. I don't think adding a few fish would really have an noticable effect. It would be a different story if it was highly stocked to grow fish for food though.