You're looking for coldwater minnows, which are like goldfish, except they won't shit up the pond and are relatively small. Suggestions like >>2365993
Experience: Parents wanted me to build a pond for them, I did, despite not knowing the extent of how awful goldfish and koi are. Mother bought about 12 medium sized goldfish, dumped them in there, bought a shitty spring filter and called it a day. Pond was cloudy and filled up with algae the next week, never touched it knowing there could be some kind of contamination of dead fish in there so it's been unclean since November 2016.
I suggested minnows since they were smaller and can swarm but my mom thought goldfish and koi were more cuter. This is the last time I helped them dealing with aquaria.