Wow

[Richard]

I have been wondering about the effect of PAR ratings through that dull space of air between the light and water. I guess going from 18" to 4" would be of great difference as is though!

No need to wonder about it. Physics has already figured it out. It's called the inverse square law. A simple way to look at it for what we care about is...

(far distance/near distance) squared = brightness

So for Mike... (18 / 4) squared = 20.25. So by lowering the lights to 4 inches from 18 you increased the light intensity at the water surface by 20.25 times.