Picture comingTiny floating fern that lives with a nitrogen-fixing cyanobacterium (Anabaena azollae) in its leaves, fragments and spreads fast, and turns reddish brown outdoors in autumn.
Check your local laws. England: sale banned since 6 April 2014 (Wildlife and Countryside Act 1981 sale-prohibition order lists 'Fern, Water, Azolla filiculoides'). Not on the US Federal Noxious Weed list (native to the western USA), but the related Azolla pinnata is. Not on the EU Union list as of the 2019 update (later updates not checked). Never throw it into ponds, rivers or drains.
14 facts checked with sources