It all depends on how much, and which species of your fish like salad, and intensity of light.
My val grow so fast, that I usually have 8" blades floating along the surface, beyond the 15" vertical plant below the surface.
They get a short period of indirect sunlight, which I believe really promotes growth, but they are also in a tank of Lake Barumbi mbo Tilapines that chew down any browning, or softening leaves.