I like lilaeopsis, common name is micro sword. it stays short 2"-3". I also accent with dwarf hairgrass, but it is a pain to work with, many small little plantlets. The thing with ground cover is that to achiev the ground cover effect you have to plant heavily and that almost neccesitates that you suppliment CO2 for good growth. Im not sure how large your tank is but 55 or smaller you can make a simple DIY CO2 system using sugar, yeast and water and run the tubing output to a powerhead.