I preferred my 400W radium 20,000K's to be 5-6" from the water. They also had glass splash guards on the pendants.
I'd run a 250W 10,000K at 7-8" with a splash guard.

I ran a 150W at 3" above the water, usually with a shield that I had to clean all the time.

I based my hight preference not on how bright they were in the tank, but on how much they messed with the temperature
Hope that helps!

-Brett