Hornwort is a species of aquatic plants that is commonly used to keep pond and aquarium water clear. It adds oxygen to the water as it grows, eating up excess nitrates. According to Jim Kelly of the University of California, "One rule that you should follow is to plant very densely. Remember that plants use up available nutrients and thereby prevent algae from getting a strong hold. If you try to save money by planting one plant at a time, you'll only grow an algae garden."
Plant your hornworts by setting them into your aquarium or pond, at least 10 inches deep. They can grow a hundred feet tall in deep bodies of water as New Zealanders have realized. The plants grow there so voraciously that they struggle to keep hornwort under control. You can grow these rootless plants as long as you have enough light.
Cut off or break off the tips of the fronds and let them settle to the bottom of the water. These will quickly grow in warm temperatures to form new plants. In the wild, these tops will break off in the winter when the temperatures fall. In the spring, growth restarts from these tops.
Thin out the hornworts when they have grown in mass in the water and are threatening to fill up the area. Grab handfuls and throw them into the compost or use a sieve. Just be warned that this movement will probably break off pieces of the plant that will regenerate into new growth, so be prepared to thin them out on a regular basis.