I'm not sure why that is too. When I move some hornwort to my pond, it stoped growing and just gone ungly but the one in nature pond kept growing like crazy. The only different is my nature pond didnt have any type of filter or water circulation but it kept the water clear (and i had a few guppies and a betta in there and they survived just fine so the water must be ok).I wish mine would. Seems like by the end of the year I have less, or at least the same as I started with.