Normally, I would say that 150W MH is fine for about 19" of depth that you are mentioning. While using a 20k bulb, you will obviously see less light penetration than when using a 10k.
I too have a 150W but my tank is 24" deep and it doesn't do enough, even with a 10k bulb.
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Mark...
290g Peninsula Display, 425g total volume. Setup Jan 2013.
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