Did you know that goldfish have no stomachs that is why they will eat and eat if you feed them often I have just recently been told this by a fish expert 'Well I never'
Read this I found it on the internet it applies to goldfish as well
Koi do not have stomachs. All their food is digested in the intestinal track so they never fill up. As food is going in one end it is pretty much coming out the other end. So you have to control how much food they eat. Not only that if you over feed you end up with fat fish and that is not good. And the more you feed the dirtier the water will become.