I'm doing the same in a 20L too! I'm using a sand substrate and root tabs and it seems to be growing well. They send out runners so yes they grow out horizontally! I used 2 tissue culture pots of dwarf hair grass and planted into clumps - you get quite a lot per pot and 2 was heaps for my 20L. I'm not using any special lighting and have had growth in one month. I believe if you used a special grow light, specially formulated plant substrate etc it would grow a lot faster but from my experience so far it seems to still be growing in more a low tech setup!