I was asked to check into this, and I believe the plant is Acorus gramineus "Variegatus", commonly called Variegated Japenese Rush. Leaves are stiff, from 30-50 cm (12-20 inches) long, arising from a thick rhizome [as in the photos]. Another variant has a yellow stripe on the leaves, it is called A. gramineus var. aureovariegatus and is otherwise identical to A. grimineus "Variegatus."
There is a form called A. gramineus var. pusillus that has leaves up to 10cm (under 12 inches) and slightly wider. This one apparently is better in cooler water (below 23C/71F).
These are not true aquatic plants, but my sources say they can last up to a year submersed in aquaria. They do better partly submersed, as in a plaudarium, with the upper half of the leaves above water.
They all last longer at cooler temperatures, but Hiscock says they are OK up to 26C/79F though they will not generally last quite as long at the warmer temperatures.