If you do do the shoplites check out Walmart where you can get them for less than $20.00 each.
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Yeah ... I wouldn't go with the white painted shop light .. for the little extra money I'd get the Diamond Plate at HD. It's a bit more versatile. If you decide to go higher light you might not have to buy another fixture. If the light is too much for your needs, you can raise the fixture or use window screen to block it some.
I used a 2 bulb white painted T12 shop light along with a single T8 aquarium fixture before I bought the diamond plate one. The setup grew low light plants very slowly .. maybe I should say it kept most of them alive, some grew more than others but it just wasn't quite enough for my 75 gal.
So with 6500k t8 bulbs in that fixture on the glass what do you have? Medium light or high? Say on a 40 breeder
Assuming it's 16" tall and 2" substrate, sitting right on the glass it would put the light at about 14", so according to the chart it would be approximately 78-80 PAR.
As quoted by Hoppy (the lighting Guru) in this thread: http://www.plantedtank.net/forums/showthread.php?t=184368
I don't believe there is any consensus about the definition of low, medium and high light. But, here is my definiition, subject to, and almost certain to change:
Low light - 15-30 micromols of PAR - CO2 is not needed, but is helpful to the plants
Medium light - 35-50 micromols of PAR - CO2 may be needed to avoid too many nuisance algae problems
High light - more than 50 micromols of PAR - pressurized CO2 is essential to avoid major algae problems
This is what some of us go by until more info comes in stating otherwise. So that would mean very high light.
Since that's an 18" wide tank you might only get about 40-45 PAR at the front/back glass. The lower it is the less the light spreads and might not spread enough on that tank height. It would be really high light in the center of the tank but only medium or high-medium at the front/back. Hoppy could answer this much better than I can. I'm just using my test numbers to guesstimate it .. refer to this thread:
http://www.plantedtank.net/forums/showthread.php?t=185893&highlight= where I tested 9" toward the front & back to see what PAR would be like at the front/back glass.