https://en.wikipedia.org/wiki/Pistia
fast growing and can easily cover the surface of the plant aquarium if left to self-propagate without maintenance.
https://en.wikipedia.org/wiki/Riccia_fluitans
Under the right conditions, the entire surface of the plant will be covered with beautiful oxygen bubbles.
https://en.wikipedia.org/wiki/Lemna_minor
Lemna minor has been shown to remove heavy metals such as lead, copper, zinc and arsenic very effectively from water at non-lethal concentrations.
2nd photo by https://www.flickr.com/photos/cjewel/
Extremely easy to grow and unpretentious plant. The hydrocotyle is suitable for every beginner aquarist
https://en.wikipedia.org/wiki/Hydrocotyle
https://en.wikipedia.org/wiki/Phyllanthus_fluitans
Strong light - the leaves will turn completely red. This rich color is most desired by more aquascapers. Low to medium light – leaves will remain bright green with a slight tinge of red on the edges or slightly spotted in the center.
Photo by: https://www.flickr.com/photos/ericinsf/