the "shimmer" is from a combination of a single-point light source (MH, sun, etc) & waves &/or surface aggitation of the water that moves the reflection of the light source around
it can be seen w/ t5 but is more prevalient w/ MH since it is a more intense single-point light source like the sun
i am supplementing my t5s w/ some leds & keeping the surface of the water agitated to give the shimmer effect