you do have to mesh it somehow. i would recommend, as an easy way out, buying a bit of soft mesh, tying the riccia into palm-sized balls, and letting it grow out of the mesh until it conceals it. you can do this by floating the bags at the top of the tank to start, and them weighing it down with gravel in the mesh once it's thick enough. you basically want the riccia bits to grow into one another, forming a more complex web that can carpet the tank bottom. starting it in small clumps and putting it near the lights at the top (riccia is a floating plant, remember) will speed the whole process up.