tank shape does matter when it comes to livestock. Large angelfish, tangs, sharks, and in general other strong swimmers do better in longer tanks where they can swim greater distances. I guess what I am saying is that a 9 or 10 inch angel fish is going to be a whole lot happier in a 120 that is 6 feet long, as opposed to a 120 that is 4 feet long and 4 feet tall. Taller tanks can cost more to light, as often only MH
lamps can penetrate the greater depth of water in these. Other than that, it really is up to you.
If you are going to make this a reef tank, you really should consider the MH
fixtures, in the long run if you retro fit them yourself it can be cheaper and you will be able to keep a larger variety of corals.
As a general rule of thumb, divide the total wats of light by the number of gallons. For your 120 gallon tank, a four bulb 80wat T5 fixture would give 2.6 wats per gallon, while twin 200wat MH
pendants would yield 4.1 wats per gallon, and you would have to supplement this with actinic PC
, T-5, or VHO
bulbs for another 100 wats or so, or 1 more wat per gallon. To keep any of the SPS
corals or clams, a very high light level is needed, like with the MH
fixtures. To keep corals that live on the sand bed you would probably need MH
or one heck of a strong PC
, T5 or VHO
setup, it really is up to you. I would only decide on what lights to get after careful consideration of what type of coral or reef you want to have. If you have no desire for SPS
, clams, or most of the LPS
corals, and only want to keep zoos and mushrooms, the much cheaper 4x80wat T-5 system should be fine.
I have also found that if you have a wooden hood that covers the tank, it is much, much cheaper (and more fun) to buy the parts and custom build your own lighting system yourself.