It depends on the plant species. Most of our plants are grown in a controlled environment green house, hydroponically. Common aquarium stem plants such as Ludwigia, Bacopa, Myriophyllum, etc and rheophyte / epiphyte plants (Anubias, Bucephalandra, Microsorum, Bolbitis, etc) are are growing emerged, hydroponically in a controlled, high humidity environment. True aquatic plants that can't grow emerged such as Vallisneria species, Elodea species, Najas species, Cabomba species, Potamogeton Gayi, Ceratophyllum demersum are always cultivated submerged, underwater.