Hornwort

Stiff, needle-thin leaves sit in whorls around a long stem, so the plant looks like a green bottle brush. It has no proper roots at all — nothing holds it to the ground, and in the water it keeps its shape by itself.

About as hardy as an aquarium plant gets: it grows in cool water and in warm, in dim light and in bright, without carbon dioxide and without fertiliser. The catch is the speed — it needs cutting back every couple of weeks, and in poor light the lower parts shed their needles and litter the tank.

Either let it float at the surface or push the stem ends into the substrate along the back wall; it will not root, but it stays where it is put. A floating tangle is a nursery: fry and shrimp hide in it, and many fish spawn straight into it.

By itself. The stems break into pieces and each piece grows on, so a single portion turns into a thicket in a month. To speed it up, cut a stem in half and let both parts float.