Try ghost shrimp first, although there not that good together since ghost shrimp are tropical and goldfish are coldwater, keep the temp between 68-70, higher is bad for goldfish, lower is bad for shrimp. These shrimp go for about 30 cents at petsmart, there about the size of your thumbnail so they will molt and grow, my lfs sells them large, about 1 inch long for 58 cents so id rather go for the 58 cent ones since they are bigger. If they are eaten its only 30 cents loss and that would mean no more shrimp, if they survive and you dont like them, set up a small tank, maybe a pico tank at 3 gallons and leave it to them, then buy similar shrimp like the red cherry.