Ceratophylum demersum.
Hornwort is one of the easiest freshwater plants to grow. This is demonstrated by its success in the wild where it has spread to every continent except Antarctica, after originating in North America. Its high tolerance to various water conditions makes it ideal for beginners, while its fast growth rate and ease to propagate means a little will go a long way. It can be used as either a floating plant or rooted in the substrate, giving you more freedom when designing the look of your aquarium. This also increases the range of freshwater fish that can benefit from it.