Native Elodea is less robust than its aggressive invading cousin form South America. The aquarium industry introduced the Brazilian species that has become invasive.
Brazilian has small, strap-like leaves (1-inch long) in whorls of 3 to 6 around a thick stem. Native Elodea is smaller and typically has whorls of 3 leaves. Occasionally small white flowers with 3 petals on the frail stalk rise just above surface.
All species forms very dense mats reaching the surface and choking out other plants. If you observe Brazilian Elodea please take time to report its location. Long term professional chemical control is required to re-establish native species.