Looking for Azolla or Duckweed.
Azolla (Azolla filiculoides) is a small, free-floating water plant. Goldfish enjoy eating azolla almost as much as they do duckweed. Azolla has fern-frond-shaped leaves, small roots and produces fall-like colors in shades of orange, red, gold, purplish and green. This coloring makes it an unexpected treat to the eye. Azolla is hardy in zones 7 to 11.
Duckweed (Lemna minor) is a perennial water cover that goldfish and koi find irresistible. This quick spreading plant will double in size on a daily basis, if given proper growing conditions. Remember to keep a sufficient amount in a sunny, fish-free area to ensure a constant supply. A sufficient amount is several cups of the plant. Duckweed is hardy in zones 6 to 10.