Floating plants
Plants that float on the surface of the water with hanging roots. They absorb nitrates quickly, create shade and provide shelter for fish.
Detailed explanation
The most common floating plants are: Salvinia (easy, fast growing), Lemna (duckweed, invasive), Limnobium (Amazon Frogbit), Pistia (water lettuce), Phyllanthus fluitans (Red Root Floater).
They absorb nitrates and phosphates from the water very quickly. They are the most effective natural filter. In shrimp and fry aquariums the roots provide refuge.
The main problem is explosive growth. Without regular pruning they cover the entire surface, blocking light from the plants below.
In planted freshwater aquariums with CO2, floats should be limited or avoided: they steal light and CO2 from the submerged plants.
In the pond, floating plants are very useful: water lilies, lotus flowers, Eichhornia (water hyacinth, invasive, prohibited in many countries).