
Lobelia cardinalis, dwarf or small form. The wild or pond Lobelia is much too big for the aquarium. There are two "dwarf" variations for aquariums. When grown underwater the plants lose all their purple/red coloring and turn light green.
The dwarf form is grown in the USA by Florida Aquatic Nurseries and can reach a height of 8 to 12". This is the common one. A not so common one pictured above is the "small form" variety imported from the Orient. It reaches a maximum height of about 4" and has more round shaped leaves.
I am only one of a handfull of people importing this plant ino the USA, and I have it for $3.79 per bunch.
Here is a detailed article about this plant written by Erik:
http://www.e-aquaria.com/des_lobelia.html
Here is what the regular dwarf looks like full grown:

Best regards
Robert