Goldfish are primarily vegetarian and need a lot of plant matter in their diet. So they should have live plants in the tank they can eat.
Duckweed is a small floating plant that most goldfish love. It is easy to grow and you can keep it in plastic containers outside in the sun. Grab a handful each week or whenever and add it to the tank for the fish to eat.
Fancy goldfish have shorter bodies than normal goldfish and their internal organs are squished up. This can make it harder for them to swim and digest food, and they have more internal problems than normal shaped goldfish like common or comet goldfish.