The short answer
Goldfish can nibble tropical flakes now and then, but they should not live on them. Goldfish do best on food formulated for goldfish — built for their plant-heavy diet, grazing style, and unusual digestion — with sinking pellets and the occasional blanched vegetable for variety.
Goldfish digest food differently
Here is the key difference: goldfish have no true stomach. Food passes more or less straight through their gut, which means they are built to graze continuously on lower-energy, fiber-rich foods rather than process rich, protein-dense meals. Tropical fish formulas are generally designed around higher protein for active swimmers with real stomachs. Fed long-term, they can leave goldfish prone to bloating and digestive trouble.
The second problem is where the food sits. Floating tropical flakes encourage goldfish to gulp air at the surface, which can contribute to buoyancy problems — especially in fancy varieties like orandas and ranchus, whose round bodies already struggle with balance. Sinking pellets, or flakes pre-soaked so they drift down, let goldfish eat the way nature intended: nose-down, sifting and grazing near the bottom.
What a good goldfish diet looks like
Start with a quality goldfish-specific pellet as the staple — smaller pellets for juveniles and small fancies, larger ones for pond-size commons and comets. Then add variety. Goldfish genuinely enjoy blanched vegetables: softened peas with the skins removed, blanched spinach or lettuce, and thin slices of zucchini, all prepared plainly with no salt, oil, or seasoning. Remove uneaten vegetables after a few hours so they do not foul the water. Duckweed and other edible tank plants can double as grazing snacks. Treats like bloodworms or brine shrimp are fine occasionally, but keep them as treats — rich foods in small doses, offered once or twice a week at most and never as a substitute for the daily staple.
Getting the right bag the first time
Pet store shelves are overwhelming, and advice varies. KinsLe's Pet Services Directory can point you toward a reputable local fish shop where experienced staff can help you pick a goldfish food matched to your fish's size, age, and variety — whether that is a fancy in a 20-gallon or a comet headed for a garden pond. Bring your questions about pellet size and feeding frequency; a good shop will happily talk you through them. Pet Services Directory on KinsLe helps you find an experienced aquarium retailer whose staff understand sinking foods for bottom-grazing goldfish.