Azolla is a very small aquatic fern. The literature is mixed about whether it is good fish food. The Azolla Event brought on an Ice Age about 49 - 50 million years ago when Azolla growing rapidly around the Arctic Ocean and then dying and sinking to the bottom when the salinity got too high sequestered enough carbon dioxide from the atmosphere to make temperatures plummet. See: https://en.wikipedia.org/wiki/Azolla_event. Some are suggesting that this might be a way to stop global warming.