The Kessil LED light and LED lights that come as “point sources” rather than as strips of lights can have a feature called “shimmer” which is where they make ripples of light on the bottom of the tank.  I’ve never really observed that with my set-up in the past, attributing that to not having the light over open water but rather shining through a clear lid that gets condensation on it for a more diffuse type of lighting.  Recently however I had to relocate the powerhead up higher in the tank to avoid getting the ludwigia sucked into it and that resulted in a lot more surface water turbulence.  Suddenly, there’s all kinds of shimmer-like effects.  Is that was people generally mean – that the effect is caused by wavelets on the surface of the water?  The powerhead alternates every two minutes beween the lowest setting, and the next-to-lowest setting, and while on the lowest setting the shimmer is visible – look on the sculpture inside the tank and on the roof of the fireplace above the tank – on the higher setting it is very apparent.  Some people like shimmer and some don’t; I didn’t even notice it until a week after adjusting the height of the powerhead.  When the powerhead is deeper in the tank there is still a lot of water flow but the surface stays relatively calm and there’s no “shimmer”.

The fish don’t seem to particularly notice or care in either case.

Videos included after the break…