As long as you acclimate the zoas properly to the MH, you wont have bleaching. And as long as you have overhangs and such that you can put frags like blues and others that prefer a shaded area, than the zoas would be fine with a 150 watter. I also remember seeing somewhere that Mike plans on having SPS frags in his tank... eddie has plans fgor a zoa-dominant tank. The 150 watter would be a much easier option.
Eddie, if you are worried about your zoas bleaching, dont worry. Nothing will bleach out. That is caused by improper UV filtering and/or improper acclimation. The frag you recieved from me may have been lighter because those couple of polyps were'nt getting direct light. I have not lost ANY color in ANY of my zoas no matter how close they get to the MH (except the blues which i moved to the end of the tank away from the MH.)
I do think that 400 watts over a 12" deep tank is overkill though... i just have to say that. 250 is understandable though.
Eddie. I know i keep goin back and forth on this, but there will be nothing wrong with a 150 watt 20,000k MH over your tank. You will just have to hang it higher in the beginning and slowly lower it. Or use pieces of window screen over the top and remove a layer every week or so. But you will have to do this with any MH that you add to your tank.
Keep this in mind too. Your MAIN reseaon for doing this upgrade is because you see the color change that takes place when I take a frag from you and put it in my tank. But consider this. there are frags from my tank that I can give to you that will change and look more attractive in your tank. It all depends on the color variation and what kind of lightin the zoas have become accustomed to.
Just rambling now,
Jesse