How long did the fish take to get fat?
Was it overnight or did it get fatter over a few months?
The food is fine. You can feed tropical fish on goldfish food if it's part of a varied diet. The goldfish food tends to be more plant based and has a lower protien content (as you noticed).
Fish should normally poop once or twice a day. My concern with your fish is the scales are sticking out and the can indicate dropsey, which is normally caused by an internal infection. There's not normally any cure and fish that suddenly swell up and have their scales sticking out from around the belly, usually stop eating and do a stringy white poop, then die. If your fish is still eating then that's a good sign.