This would be a GREAT budget light for your tank. http://www.thatpetplace.com/pet/prod/236417/product.web
if your tank is 55 gal or less this puts you at 2 wpg and T5 is stronger / cooler light that penetrates deeper in the tank. I much prefer T5 HO to PC lighting any day for many reasons.
The downfall for this fixture is that it doesn't fully maximize the lights by only utilizing one reflector for both bulbs. Better fixtures Tek or others by Sunlight Supply have very highly reflective individual reflectors behind each bulb, spacing them out so that you get more light directed into the tank and lighting a broader area. However for your 55 this would work fine if your 18 inches is up and down and your 15 inches is front to back. If your 18" is fron to back, you would do better with a fixture with 4 T5 bulbs in it or getting two of these fixtures.
However, if you can pay double what that cost, you might be better to get this. http://cgi.ebay.com/SUN-BLAZE-T5-HO-...QQcmdZViewItem
This has individual reflectors and is very good.
This is a cheaper version of my favorite light to use over a fresh water planted, the TEK
the ballasts and endcaps and reflectors are the same as TEK I think.
The reflectors might be TEK 1 reflectors, there are newer better ones that are not in this fixture, but it shoud still be above 90% effective, where the TEK 2's are about 95% plus.
Another draw back to this light is that it has only one power cord and one on off switch.
The Tek for aquariums has two power cords with two on off switches.
Basically this allows you to customize your lighting set up automatically with timers.
I run two of my 4 lamps for the whole 9 hours that my lights are on, but only run the other two for a 3 hour period in the middle for a "mid day blast" representing "high noon".
all 216 watts of T5 with 20,000 lumens would be just too muc for you to run all day with out getting algae.
Of course you could hang the light up high, very high, or you could shorten your photoperiod to say 6 to 7 hours or you could only house a few bulbs (which defeats the purpose of getting this fixture) or if you are hangd with electrical, you can, and I have seen threads of some who have, add a cord and or switch to the fixture so that you can automate the process.
anyway hope this helps a little, I think your main question was how to light your tank, so there are some opinions and ideas