I would go for 250W if at some point you want SPS or Clams, but I would think with a tank hight of 18" you could get away with 175W (sorry Last post I didn't read tank spec).
I have my MH on a canopy that I had the guys at Animal House commision for me. In hind site I could have built it myself but I'm very please with the work. My MH site about 14" above the water (2X250W)
J
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