Best substrate for Fancy Goldfish?

Large round smooth stones or fine sand. Gold fish are going to eat small size gravel if food falls to the substrate so you either want it too big for them to eat it or too small for it to matter. Make sure the rocks are smooth though because otherwise it can cut up the goldfish when he lays on the bottom.

your friend should totally train their goldfish to play soccer

I’ve always used smooth glass ‘stones’ and marbles for my goldfish. They’re pretty, especially with a light on. Plus they make it easy to see if there is dirt to vacuum up.