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Not sure what plant it is but I would not get it for your aquarium. Most plants you find in the wild will contain some unwanted pests/diseases. Also that plant has been grown emersed and would probably need to change to a submersed form (if it has a submersed form).
That looks like alternanthera philoxeroides aka alligator weed. It's primarily emergent but can live submersed, although I'm not sure if it can permanently. I kept some of this a few years back and I recall it being a bit buoyant from the large hollow stems. Found this photo on Flickr