Hoppy, you've been such a good help with lighting before, let me ask you something as you'd be better interpreting the graphs and giving advice than anybody.
I have a 29g tank, and am looking for low light, or more specifically, the highest light possible to get away with only using excel for CO2. For the room it's in, if I used one "good" metal vertical housing mounted about 3' above substrate, what bulb wattage would I need to hit my target? What if I used two housings? Finally, what if I just set a housing or two directly on the glass?
I know it's a lot of questions, but I figure you might know an answer in a minute, as opposed to me looking at the graphs for hours and ending up with the wrong answer anyway!
Edit: I figure I'd want to use 2 fixtures (to get better spread), in which case just tell me which scenario you'd suggest and what to get, as I'll do whatever you'd suggest.
Edit 2: The more I look at the one graph, the more I'm pretty sure that if I'm at 15" (on top of glass), I'll surely have too much light with two fixtures. And if I am 36" above (can mount to something at that height easily) I'll surely have too little light no matter what. So, I will find a way to mount at any height if you can tell me at what height and bulb wattage you think a fixture system would work best.
Rush, I think you can use 23 watt bulbs, with the reflector rim 2 inches from the top of the tank, and 2 inches of substrate. That gives you room for air circulation to keep the bulbs relatively cool. And, you should have 40-50 PAR light intensity. If any algae problems pop up, you could raise it another inch, or switch to 19 watt bulbs.
I ran 2X23watt bulb at 17inch from the substrate for sometime(similar fixture but 10" wide). I had lot of green string algae and sometime black algae spots on the glass. I was doing DIYCO2 but i wasn't changing them very often. DIY CO2 and 13watt bulbs seems to be fine.
Warning: Find a good way to hang the fixture otherwise it fixture will look ugly.
GAT, good to hear from someone with experience with the nearly exact setup. I already ordered the 23 watt bulbs, but I will count on starting them off a little higher. And yeah, they definitely aren't exactly the greatest things to look at, but my gf is very crafty, so I plan on asking her to find a way to spiffy up the outside of the fixtures to make them look nice.