[Ron Reefman]

Yes, light transmission is an issue. In general I assume most reefers work to keep their water about as clear as possible and the salt creep/evap cleaned off the light fixture. That's part of the reason nobody can give anybody a perfect answer about how to set their led intensity. Fixtures are different, water clarity is different, height off the water is different, depth in the tank is different...

But look at it this way, if you run MH or t5 florescent you don't have much, if any, control over intensity other than placing screens and such in the way.