They are called vallis, they come in different sizes so beware Vallisneria Gigantea it grows up to and beyond 120cm. Also not all Vallisneria have the same requirements so be sure to check out their own special requirements, light, co2, fertilisers, temperature etc.
All of the Vallisneria need good lighting and grow much better with the addition of some form of fertiliser. But I must say they do do better with the addition of CO2 as this then helps them create a greater oxygen output. Lighting should preferably be 75W/100litres.
That light is only 20w and no where near enough for good plant growth. When the tank is planted as heavy as you are attempting, a lot of the light will be blocked by the upper plants and the lower ones will miss out and begin to die, thus in turn polluting the water. Some how you neeed to get that wattage up to around at least 40w by adding another tube using a separate light unit capable of handling 20w. Something like the Hagen Glomat single Starter Unit 20w @ ?18.19