He shouldn't be using feeder goldfish, they are often diseased and very weak because of the conditions they are kept in, they are not bred to live long (as they will just be fed to other fish) so I doubt they would live through a cycle. I would reccommend zebra danios, you can get them for fairly cheap (often under $1) and they are extremely hardy. I group of a minimum of 6 is what I would reccommend. Another bonus with the danios is that you can keep them with practicly everything so if he didn't want to he wouldn't have to rehome them (with the goldfish, you would as they would get way to big for that size of tank).