Never used them, but I have used HID lights with my coral reef tank. Mh only...HPS doesn't extend deep enough to work, and most usable spectrum is dispersed in the top couple of inches of the water column.

A couple of things to ponder...

Different wavelengths refract differently.

Light thru a glass (or a curved vessel of water) will have an uneven spread of spectrum.
For instance...red light effectively extends down a couple of meters in the ocean, max. Yet blue light travels for what...100 plus meters or so...?

How much light is caught between the two walls of the cooler-tube?
Would be interested in PAR and spectrum readings from it, tho.

What happens when/if a failure occurs?

How would you cool the heated water for re-use?