Goldfish love to nibble on and eat plants and algae. Plants provide shelter and a place for the fish to hide. Plants also provide oxygen. A great inexpensive plant that goldfish seem to love is Anacharis. It is generally sold in bunches. You can float a few pieces, push the base of the Anaharis into the rocks or attach it to the tank's glass with suction cup devices made for aquariums. It allows them to eat it without having the mess of the plants decaying in the gravel bed. As it dies off be sure that you remove any of the dead/decaying plant, though.
If this information was useful to you please "click" accept. This allows me to be paid for my time and efforts. Bonuses welcomed for job well done. Thanks so much!
aits@mchsi.com