Blyxa is a great grass (personal favorate when it comes to grass), which is weird since it is a stem plant... Both japonica
(great foreground plant maxing at around 6" assuming you do not get it terrestrially grown) and aubertii which grows to about 60cm (about 18") I think in a 150 these would make a great contrasting fore and mid ground plant (I say mid-ground cause you have 30" to work with). Some dwarf micro swords would work well for the very front, (assuming you have the lights), or a nice type of moss. Riccia might also be a possibility as long as you are willing to commit the time to weigh it down.