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I like feeding pellets best (the sinking ones) because watching the fish forage for them is fun and it also makes it so they don't gulp air. I recommend Saki-Hikari supplemented with Omega One, Algae Tablets, and Duckweed. You should grow duckweed for sure. It floats on the surface and the goldfish like to go up and eat it as a supplement.
Floating pellets work well too but the fish can gasp in air which is bad for their swim bladder.