If left alone for any amount of time this stuff will spread like wildfire. You're goldfish will love you buying some of this. It's a great floating plant and serves as excellent cover for baby shrimp. Be carful not to let it cover the whole top of your tank or you may deprive the rest of your plants of light. It's a great self sustaining food supply for most aquatic animals. But a scoop of our duckweed today!!!