How tall of a tank will a MH 150Watt work for?

[ner'T']

HI,

I have a 150 Watt MH w/ 20K bulb. I want to put it over a colum tank. I'm going to get the a Marineland 47, or 37, Column tank.

The 37 is 24" tall and the 47 is 30" tall. I realize the light might be too little by the time it reaches the bottom... Is 30" too deep for my light?

My preference is the 47 Gallon Tank.

Does anyone have any input?

Thanks for your help!