PC lighting is generally said to be good for tanks under 22" deep, deeper than that is when MH, VHO, or T5 HO should be considered if you plan on keeping anything but low light corals on / near the bottom.
Personally, I've found that I have a hard time keeping anything but mushrooms on the bottom of our 90g tank, which is 24" tall empty. Between the deep sand bed and the small distance I raised our PCs to allow better heat exchange, the top of our sand bed is 21.5" from the lights. The placement of corals in our tank is clearly skewed towards the mid- and upper- parts (tried some of them lower in the tank, but they didn't like it). I've become tired of how empty the bottom of our tank looks and my wife doesn't like most of the mushrooms we've seen locally (glad I talked her into the ones we already have), so I did some research and that's the biggest reason I'm building T5 HO lights into our canopy.
For your specific situation, Wrench, it all depends on what kind of corals you want to keep. Your lighting will be ok for most soft corals and many LPS. You might even get away with a SPS or two that aren't as light demanding if they are placed right in your tank. If your tank is 18" tall and you have a 4"+ sand bed, most soft corals should be fine in the lower portions of your tank in terms of lighting.
Personally, I've found that I have a hard time keeping anything but mushrooms on the bottom of our 90g tank, which is 24" tall empty. Between the deep sand bed and the small distance I raised our PCs to allow better heat exchange, the top of our sand bed is 21.5" from the lights. The placement of corals in our tank is clearly skewed towards the mid- and upper- parts (tried some of them lower in the tank, but they didn't like it). I've become tired of how empty the bottom of our tank looks and my wife doesn't like most of the mushrooms we've seen locally (glad I talked her into the ones we already have), so I did some research and that's the biggest reason I'm building T5 HO lights into our canopy.
For your specific situation, Wrench, it all depends on what kind of corals you want to keep. Your lighting will be ok for most soft corals and many LPS. You might even get away with a SPS or two that aren't as light demanding if they are placed right in your tank. If your tank is 18" tall and you have a 4"+ sand bed, most soft corals should be fine in the lower portions of your tank in terms of lighting.
