Nitrogen Cycle: Ammonia - Nitrite - Nitrate
Important to understand this on back of your head especially with goldfishes. The more we feed them, the more and faster the water get polluted and become harzadous for them to live in. So biofilter is compulsary or the harder choise is more frequent waterchange. Other option is to use green water or plant which recycle or consume nitrite & nitrate to grow.
Here some detail for reference:
http://www.thetropicaltank.co.uk/cycling2.htm

Thanks to Sean Evens, Tropical Tank