If I could have only one species of plant in my tanks... it would be Java Moss.  With its thin thread like stems and tiny leaflets, it forms a dense mass that absorbs nitrates better than most larger plants while providing perfect hiding places for vertebrate and invertebrate fry to hide.  In addition, it can be grown on submerged branches to look like a miniature tree growing in your aquarium.  Protozoans live and feed around it providing an additional food source for your fry.  Then to top it off, its bright green color adds a nice contrast to the multiple critters living in the tank.  Java Moss is the best addition to any aquarium breeding dwarf shrimp.  Neocardinia shrimp thrive in it.  A must especially if you need to over populate a tank with critters.  I'll give you a large bag of Java Moss so you can populate multiple tanks!