i personally would mix a few different kinds of vals to get a nice effect. i have about three different kinds in my tank and i love the contrast of the darker leaves next to the bright green of another val type. all but one of my vals are along the sides, back corners, and some of the back of the tank. makes the ends of the leaves 'pool' nicely on the water surface on either end of the tank, away from filter flow, as my filters are nearer to the middle of the back of the tank.