Best oxygenating floating plant

I've used Salvinia with Anabantids (Bettas) in small tanks
quite successfully. I was wondering if Salvinia are very good
oxygenators for water since they float on the surface and their
leaves are actually in the air. It could be that these plants
get more ammonia from an environment if they _don't_ oxygenate 
the water. They get all the CO2 they need from the air and have
little need to protect their "roots" since they don't penetrate
into the substrate.

[Q] : What is the best non-rooted oxygenating plant for use with
small (say 2 liter) containers of fish like Killies????

Thanks for any suggestions!