Giant Vals (Vallisneria gigantea)
From The Aquarium Wiki
Synonyms, alternative common names and variants
V. americana - may just be very similar
Giant Vals, Jungle Vals
This plant grows multiple strap-like leaves from crowns, and propagates readily via stolons (runners) which will produce new crowns every few inches.
Growth is very strong, given at least medium lighting and some fertilizer. The maximum height is unknown to this writer, who regularly cuts leaves up to 8 feet long back. Once the plants have reached the surface of the water, growth becomes much stronger due to their improved access to light. The maximum width listed is for individual leaves. While the crowns stay relatively compact, as they propagate the maximum width at the substrate could be said to be "the size of the tank". Also, a single crown left untrimmed could easily use up the entire surface of the tank.
Under some conditions the plant will also produce thin flower stalks with a spiraling appearance.
As a plant native to soft acidic environments, these should be their keeper's goal; however, this writer does not know how hard or alkaline the water can be before it causes a problem for them.
Copper in a sufficient concentration to kill snails (about 1 ppm or so) is very hard on this plant, but can be remedied in time if stress is observed by water changes.
Be prepared to engage in regular trimming and harvesting unless you want a tank that is pretty a V. gigantea-packed container.
Carefully managed (allowed to propagate in to the left and right in the rear of the tank), a healthy stand can completely block the view of the rear of the tank and any unsightly hardware.
There is a "dark red" variety.