Salvinia minima is one of the more common floating plants in the aquarium hobby. It's a fast growing plant that can be used to provide shade, inhibit algae growth, and pull nitrates from the water column.
I've found that midwater swimming fish really love to hide in it. It's a beautiful addition for your aquarium big or small.
$3 for a handful ziploc bag portion (covers 5-10 gallon tank).
$6 for double portion (covers 15-25 gallon tank).
