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You definitely don`t want to be putting vallis in your foreground.
Grass like plants to consider are Eleocharis parvula, Eleocharis acicularis (I consider this more of a mid ground plant), Echinodorus tenellus (get Tropica if you want it to stay low) or Lilaeopsis brasiliensis (mine is growing in to more of a mid ground height, but it can be mown like grass to keep it down).
Utricularia graminifolia is an excellent choice for a really low grassy look.
dave.