Family - Amaryllidaceae.
African Onion Plant - Crinum natans - Crinum aquatica.
Architectural plant of somewhat rigid appearance. Great for larger aquaria. Depending on the size of the aquarium, suitable as a solitary or background plant. Not hard to cultivate but hard to find in shops. Normally available as part of Extraplant's standard range. Make sure not to plant the bulb too deeply - if it is buried to its waistline you've got it about right.