well, i cannot speak for other posters, but, i think maybe what the message was, not that you cant build your own fixture, but rather, that you have to spend some kind of money to at least get the proper parts if you dont have them already.
now, i dont know salt water tanks, but i did some hunting, and found that the lumens and color temp and wavelenth is only available from the lights they spoke of, or from some swirl type bulbs, the big brothers of what i use, and even those arent "cheap"... a 8k+ kelvin swirl thats a 65 watter seems to be rather expensive even in a 4 pack. the small craft lights i use were 20 bucks for 4 of them, but they are only 14 watter's...
anyway...my suggestion is ask the salt water guys the type of light you need, and if you can get away with a swirl type bulb set up, go for it. if not, save your money and get the right stuff!