# Lighting Confusion - What Should I Do?



## appalachianfeet (Dec 12, 2011)

I am finding differing advice on lighting everywhere I look. Maybe that is because there is no right answer, but I'd love input to help sort out the following:

I'm trying to plant a 90 gallon tank "El Natural" using two 48" sit-on-the-tank light strips I already have. Each light strip holds 2 fluorescent bulbs up to 40 watts (T12 in one, T10 in the other -- I suspect these are interchangeable with other sizes?) No CO2 injection or fert regimen so far (prefer not to for El Natural).

Because it is a deep tank (24") I have put in raised planters (potting soil with creek sand over the top). This means the lowest plants are 17" from the surface, the next are 14" from the surface, the next level is 11" from the surface, and the highest level are small planters 7" from the surface. I also have a section of floating hornwort near the filter intake.

I posted a photo of it (along with all the tank specs) in the El Natural forum (moved the hornwort after photos were taken): http://www.aquaticplantcentral.com/forumapc/el-natural/82030-90-gallon-el-natural-i-hope.html#post616840

I have these new bulbs and am confused about which ones would be best:


2 GE Ecolux T12 cool daylight 40watt, 6500K, 3050 lumens
2 AgroBrite T12 grow lights 40watt, 6500K, 2325 lumens
2 GE T8 daylight (cool, natural light) 32watt, color temp 6500K, 2700 lumens

I am particularly confused about watts vs. lumens. I thought the general rule had to do with watts per gallon, but apparently a 32 watt T8 is as good as a 40 watt T12? Also confused about color temperatures and what plants prefer.

*Questions:*
Will any of these bulbs work? Will putting the plants close to the lights help with the low-tech & tank depth issues? Does "watts per gallon" count if I'm not trying to plant the lower portions of my tank? Will the length of time my lights are on make up for the intensity at all? How do I figure out "low," "moderate," and "high" lighting if I have plants at different levels in the tank? Is it still the same even with the raised planters? Can I just rely on watching my plants for growth/algae to gauge the results or will that be deceptive (right now the water sprite is growing quickly and there is obvious growth on most the species I put in).

I really don't have the money to go buy new light strips right now -- even inexpensive shop lights (though maybe in January). I'm really hoping to pull off a low maintenance planted tank with my current equipment.


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## aquaman555 (Mar 22, 2011)

appalachianfeet said:


> I am particularly confused about watts vs. lumens. I thought the general rule had to do with watts per gallon, but apparently a 32 watt T8 is as good as a 40 watt T12? Also confused about color temperatures and what plants prefer.


I don't see why any of those bulbs wouldn't work. People use varying color temps., but 6500-6700k works fine and some people use up to 10k and some use down to 5000k.

WPG is an old rule of thumb, I still use it to some what extent...however you have to be comparing apples to apples though and that is where the confusion comes in. Basically watts is the amount of energy the bulbs will use to be powered, lumens is the amount of light the bulb puts out. For instance metal halides put out more lumens per watt than fluorescent tubes, so therefor we couldn't say 100 WPG of fluorescent is anywhere close to the same as 100 WPG of metal halide. Because the metal halide is more efficient and puts out more light per watt.


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## Michael (Jul 20, 2010)

Your confusion is completely understandable!

The "watts per gallon" rule was devised back when all we had was T12s and T10s. So in your case, it still applies. . .sort of.

I calculate 4 tubes x 40w = 160w / 90 gal = 1.8 watts per gallon. This is in the correct range for Walstad tanks.

Now, about lumens. Lumens are a measurement of how bright the light looks to our eyes, not how useful the light is to plants. Photosynthetically Active Radiation (PAR) is the measurement of light intensity that makes a difference to plants, and you aren't going to find that measurement on any package. The only way to know the amount of PAR for certain is to use a special meter.

Color temperature is similar--it is a measurement of how the color of the light looks to us; cool, warm, daylight, yellow, white, blue, etc. Again, the color temperature means nothing to plants. So pick the one that pleases you.

Plant grow lights (your AgroBrite?) put out a lot of light in the red and blue parts of the spectrum because these are the parts that plants use. The color of these lights can be unpleasantly purple to our eyes, so while they are good for plants we don't like the way they look.

To boil all this down, definitely use the 2 AgroBrite tubes, because they are good for the plants. Then use which ever of the other tubes looks best to you in combination with the AgroBrites. The 6500K tubes are not worthless--they do contribute light that is useful to the plants, just not as much as the grow lights do.

Moving the plants closer to the surface definitely helps. PAR falls rapidly as depth increases. There is no way to know for sure how much light you have without a PAR meter. 

But don't worry! Many beautiful plants have been grown under lights like yours. If the plants seem spindly and pale, increase your photoperiod and/or move them closer to the surface. If you have algae problems, decrease the photoperiod. Experiment, and you can make it work.


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