I returned to fancy goldfish keeping last fall after a long break, so take what I recommend with a grain of salt. Also know there is a TON of online help, so feel free to double check my info. I've done tons of research and have also made mistakes in the past and learned from them.
-Moist food is best: Rapashy gel, homemade gel, bloodworms, soft-cooked veggies (peas, broccoli, carrots, etc). I feed 3 meals per day and at least 1 meal is veggie.
-Small pellets are best vs. a bigger size. Think of it as swallowing a large hunk of starchy food for a person. Small pellets won't swell as big as a large ones - we all know our goldies are piggies.
-Last but not least, most high quality fancy goldfish are imported from Asian countries and are bred/raised in warmer conditions than most "how to" books recommend. IOW, 62F - 72F water temp is too cool and digestion and metabolism is slower at these temps. The food can sit in the digestive tract and become problematic for the fish. If you watch some of the YouTubers who have beautiful, large fancies, their tanks are at 78F - 80F!
Good luck to you.