Personaly I think 400 watt bulbs would be a little overkill on a 18" tall tank.. my recomendation would be 250 watt.
you are going to need 2 bulbs on that tank no mater what (good rule of theumb is one MH for every 2 feet in lenght) you can squeze 3 feet if you can run the bulb a little higher, but you are getting dimmer at the edges at this distance.
the reason I recomend 250 watt over 175 watt, is for bulb options. (if you already have the 175 watt stuff then go with that, but if you have to buy new stuff then go 250 watt)
I have a 175 watt on my 24" deep tank and I am getting awsome growth out of SPS, but I am upgrading to 250 watt stuff so I can get the better type of ballast and also run different bulbs.. if you go to 400 watts I personaly feel the move is for looks not better preformance.. Ie.. you want to run the radiums (which is another cheep option, if you like the "Blue look" and you have the room to mount them a little higher)
Lighting is a big personal thing.. I personaly belive that a lot of us are putting more light than we have to into our tank and then atributing coral growth to the larger lights whare realy we could get the same growth and color from lower watage lights.. I look at digitata in my tank, and stuff on the oposite end of my tank at a depth of 24" below the PC's is growing just as fast and showing just as much color as stuff on the other end 6" under the MH. I know that stuff will grow under a 60 watt bulb but my point is the stuff getting 10X the amount of light isn't growing any beter/faster.
ok going to stop befor I start rambling on here.. it is a huge topic with many different directions you can go in
Steve
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*everything said above is just my opinion, and may or may not reflect the views of this BBS, its Operators, and its Members. If cornered on any “opinion” I post I will totally deny having ever said this in a Court of Law…Unless I am the right one*
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