OFFER: Oxygenating pondweed (Portslade by Sea BN41)
DESCRIPTION
It's called Hornwort (Ceratophyllum demersum), and does a great job of keeping the water in my tiny pond fresh. I just removed about half a bucket-full as it had outgrown the space, and can leave it outside my house for collection (but bring a waterproof bag to take it away).