If you use a 400watt on one side and 175 on the other you will see a huge difference in the amount of light per side. IMO the replacement and on going costs of 2 250's would probably be less than 3 175's. Really the only difference would be power costs. If you could buy a second 400 and run 2 that would be awesome on your 90 if your temps would allow for it. 175 My results weren't all that great for acros but most SPS would do pretty well in the higher areas of your tank.
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Jesse
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