Don't keep a goldfish in a five gallon tank, please. It's simply too small no matter how clean you keep the water.
I would use the 5 gal to grow duckweed and other floaters to feed the goldfish in a bigger (much bigger - 30 to 75 gallons and up) tank. In the goldfish tank itself, I would suggest Java ferns and very large Amazon swords or crypts. Floaters in the tank are great, but keep backup tubs in case they all get eaten up. Goldfish do not nibble. They eat, and eat, and eat. They're a force of destruction to be reckoned with.