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So I read the whole spiral compact thread and have a couple of questions. I really like the look of pendant lighting, and from reading the thread it looks like verticle orientation is the way to go.

I am planning a low light tank. It will probably end up with co2 after reading tom barrs comments on how providing co2 allows your plants to make more efficient use of light.

It will be an 18" tank, and i want the lights to sit ~4" above the water surface. So with substrate figure 20" hanging distance. I am trying to replicate the performance i saw in 18" tank with 2 t8s on top, hopefully with same or less wattage (64 total).

So, what wattage cfl would i be looking at to get low to moderate light at about 20"?

Can I use a lower wattage bulb with a tighter reflector?

How many, and what size reflectors to get good coverage? From what I understand, all else equal, a narrower reflector should give more penetration, but less spread. I am looking to fill a 15" tank front to rear.
 

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I would do 4 x 13 watt bulbs or 3 x 23/26 watt bulbs. 6500K is the best for aquatic plants when it comes to spiral CFLs.

Vertical mounted is best. Tighter cones are good for getting more intesity out of your light, but the trade off is less distribution of light over the tank.

I would look for 5 inch or 8 inch reflector cones.
 

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This is one of the charts Hoppy made. It's helped me a ton. If you hang the lights 20" over the substrate with a 23 watt cfl, that should give you ~ 35 PAR per bulb (assuming you hang it vertically and use some kind of aluminum (clamp lamp) reflector). That's medium light. If you go with 14 watt cfls, that'll give you ~20 PAR each....right in the sweet spot of low light. Of course this isn't exact and you'll have some light spill over from each fixture, but it gives you a general idea of the amount of light each bulb is throwing down.

I use 2 fixtures over my 10g, and I've seen people use 3 over they're 20g long to get good light distribution. I'd guess you need to use at least 4 to cover a 48" tank, maybe 5.

5 clamp lamps and 5 cfl bulbs is gonna run you about $60. Plus however you decide to hang it.

You could always try and hang a 4 bulb T8 shoplight fixture. Maybe one like this . Probably around the same price. I think each bulb will give you ~10 par at 20". So a total of 40, kinda like the cfls. Just giving you some other ideas.

 

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great chart, didn't know that one existed!

I think I may have misspoke earlier. you'd probably want 5 x 13 watts, that would give you around 1.16 watts per gallon which is pretty low (usually 1.5 to 2 watts per gallon). PAR is more accurate measurement though so If you want to avoid CO2 then you want to aim for the low light sweet spot that BlueJack mentioned.
 
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