Have a read of this to:
http://www.fishlore.com/Profiles-Goldfish.htm
The trouble with goldfish isn't really thier size as much as they are huge waste producers that will pollute a small tank very fast. In all honesty with 6 in a 15gal you are lucky they are still alive. With an eventual size of up to 12" a 15gal tank is also physically too small for goldfish to feel comfortable in. Read the
nitrogen cycle links in my previous post. Knowing about the Nitrogen Cycle is the single most important thing you can learn aobut fish keeping & it will also help you understand why I say your tank is too small for goldfish.
Sorry if I sound harsh. I don't mean to be.