A few months ago someone reported PAR measurements with and without a glass top on the tank. The light loss due to the glass, which wasn't really clean, was about 15% as I recall.
Thanks for the info Hoppy! You are the light guru. lol I really don't think I can go with the t5ho right now. They are too dang expensive. At least all the ones I looked at. Since I would need 2. If I could get by with 1, then I would probably do it. But with 1 fixture I don't think it would get enough spread on that wide of a tank. HOwever, since it would be way up in the air, it just might.
One nice thing about using low light is that even if you do decide to inject CO2, you can use much less than the typical 30-35ppm medium and high light tank owners usually strive for. A five or ten pound tank might last a year or even longer between fillings.
The 'scape you posted is mostly moss and Java fern which are low light staples. Anubias would be a good choice also, and is sometimes problematic in high light tanks. The majority of cryptocorynes do well in low light also, though they grow so slowly in those conditions they might seem dormant.
Personally, I like the idea of rocky moss tank similar to what you posted. Something like a Fluval 405, or Eheim 2217 (flow is lower so maybe run 2) or the Rena equivalent would be plenty for filtration, and can be used as your CO2 reactor since your injection rate will probably be on the slow side. You could keep shrimp in the tank, and with nothing but moss, java, and shrimp there would be no need to heat the tank. With low light, low CO2 injection levels, low bioload, and cooler water, the environment wouldn't be very conducive to algae growth when compared to the typical aquarium.
Low light would require much less fertilizer if any at all. Moss barely needs any nutrients when the light levels are low, and rarely needs trimming. An army of shrimp and good water flow should keep it from accumulating detritus.
Whatever you decide to do, be sure and keep a tank journal so we can all watch your tank progress.
Add: I would go with the 40B also. 12" is too shallow to get that lake, stream look. Everything ends up looking "swampy" in my opinion. (Not that there is anything wrong with that look, but I don't get the feeling that is what you're going for.)
Sharkfood, thanks for the reply. I was curious how much co2 would help in a low light tank. Would it be worth the investment? I think the main reason I would want it, is so I can have a nice ground cover like hc or something. I know this probably sounds dumb, but both of my tanks now have hob filters. I"m a little scared of the canister filters. I'm afraid of coming home to water everywhere or making a mess during water changes. I don't know exactly how the connections are so, my paranoia, might just be my ignorance of them.
As for plants, I'm leaning towards needle leaf java fern, weeping moss or something similar, mini pellia, some val nana or something that looks similar, and a carpeting plant. The carpeting plant probably isn't much of an option though without the co2.
And after thinking about it... the 40 breeder seems to be the best choice to scape with.