I can't see any "spot lighting" effect from the individual LEDs, so I decided to look for it photographically. Here is a normal photo looking down at the substrate:
I can't see any spotlighting on that, so I tried cutting down the exposure, jacking up the constrast, and cutting down the brightness, to see if that will reveal any spotlight effect:
Even with that, I can't see any spotlighting. From that I think it is safe to say that a 3 inch on centers spacing of the LEDs works very well, at a distance of 20 inches or so. Perhaps at half that distance it might show spotlighting, but I can't tell about that without making the water cloudy so it shows the cones of light from the LEDs.