48" light on 72" tank?

Generally speaking you get what you pay for. That's not to say you can't have a nice light system for less just realize that some things will be compromised. This is my understanding as far as aquarium lighting goes. The major difference between lighting systems is their ability to produce light and how many watts they consume while producing said light. The more expensive the set up the higher the amount of light produced which increases it's ability to light your aquarium evenly and in the case of plants helps make sure the appropriate light spectrums penetrate the water far enough at a high enough concentration to be as beneficial as possible for the plant as long as the appropriate light is used.

It can get pretty confusing. There's Lumen which is loosely the measure of the amount of light emitted per second by a source. So the higher the Lumen rating a bulb has the more light it produces. Then there's luminous efficacy which is loosely how much light is produced in relation to how much energy is expended producing said light. Then finally there's K(Kelvin) which is a measurement used to describe a specific hue of a light source.

Unfortunately when it comes to aquarium lighting many times the Lumens of a bulb is left out of the description. You kind of just have to know that generally LED's and T5's produce more light then T8-T12 Fluorescents and do it more efficiently by producing more light per watt. For instance lets say you use a 15 watt T5 Fluorescent to light your tank for someone else to light the same sized tank just as efficiently with a T8 they would need to double the wattage of the light. Again the amount of light produced basically effects how far the light will penetrate the tank and to what degree. I'm sure you've noticed if you've ever taken pics that pics at the bottom of the tank are way less lit then a pic taken at the surface. The more light produced by a bulb the more light will reach the bottom and can give your tank a more evenly distributed look light wise. It's also beneficial for plants as the more light reaches the bottom the fuller the spectrum that reaches the plant.

Now that's not to say you can't get a nice color popping look using a cheaper system just realize the colors will only pop the closer the fish is to the light source with a cheap light system. So you can get a decent look by using 2-36 inch Aqueon Deluxe Single Strip Lights($50-$55 each) and swapping out the standard bulb they come with for 2-10,000K($12 each) replacement bulbs. The light will be more white blue so the fish should look more natural instead of the yellow and green tinge the standard bulbs can give off. I kind of fumbled over myself with that description trying to be concise which is not my strong suit but I hope it helped a little.

Here's a example of 10,000K T8 lighting on one of my tanks. You can see how well lit the parts of the video are when the fish are higher in the water and how it gets dark and bluish it gets the closer I vid to the bottom. Better lighting like T5 or LED's etc. would have made my the vid of the lower sections look better.
[video=youtube;3W1w5FeTg50]https://www.youtube.com/watch?v=3W1w5FeTg50&list=UUnW1FQH99_jIp037hv_mYKA[/video]