Yea, that practice of giving goldies as prizes condemns most of them to death. It's a shame. People seem to have the idea that goldies are easy and that you can keep them in a 1 gallon bowl with no filtration which is totally bogus.
My goldies got really floaty poop when eating flakes.
Flakes caused my big moor to flip over.
My best guess on flakes is that they contain lots of grains which get turned into gas in the GI tract.
The flakes float to some degree, so people think eating from the surface causes fish to get too much air in their GI tract.
Some people also recommend pre-soaking pellets or flakes before feeding.
Personally, I think feeding flakes and pellets exclusively is a bad thing.
If you look at the ingredients you'll see fish or shrimp meal and grains/soy and added vitamins. feeding that all the time would be like you or I eating fish sandwiches all the time. meat and starch. not healthy and not even close to a natural diet.
I feed my goldies shrimp or krill and veggies (peas, green beans, lima beans, zucchini, etc) defrosted and torn into bite sized pieces, along with home made gel food. Feed em kiwi from time ot time for the vitamin C content.
If your biofilter bugs aren't up to speed yet, go light on feeding for now.
I have to get nosey here.

Please tell us about your tank.
How many fish of what sizes?
How much water does it hold?
What filter are you using?
What water conditioner?
How do you maintain your tank?
Happy water is the very most important thing in keeping goldies healthy. They're very messy fish.
During the first month or so after a new tank is set up, there are a set of bacteria that have to get up to speed to convert the ammonia that fish produce (toxic) into nitrIte (still toxic) and then into nitrAte (less toxic). It's really important during the first month to test your water and use partial water changes to keep ammonia and nitrIte low till the biofilter bugs catch up. Do you have any water tests?