Can goldfish eat cornflakes?

No. Grains are not part of the natural diet of fish. Corn flakes tend to contain a lot of sugar and salt too, which isn't part of the natural diet of goldfish (and you shouldn't eat too much of it, either).

It's better if you don't feed him at all, healthy goldfish do fine fine without food for a few days (useful to know if you have to leave for a few days).

However there is a number of veggies goldfish can eat (and should be part of their diet too). E.g. cooked/blanched peas (deshelled), zucchini (deseeded), spinach, carrot, romaine lettuce, broccoli stalks etc. Cook/blanch them in plain water, don't add any oil or salt or spices or anything, really. Peas and spinach tend to be favorites among fish, probably because they are used in commercial fish foods too.

Don't give them anything that too acidic or that has high sugar content.

Nooo don't feed him that. There are content in it that is very unhealthy for goldfish or any other fish in that matter. Typically, goldfish will not die if they don't eat for one or two days. But if you really want to feed your goldfish, you can feed them boiled, and peeled peas. Hope I helped! Good luck and God bless! :)

I wouldn't, but if you have to he will probably be okay. But just get him regular fish food ASAP.

No. Store-bought cereal is very high in salt.

No!!