# Computing light on smaller tanks



## Deni (Jul 6, 2005)

I'm in the planning stages of setting up a 12g and a 6g. I've been doing a lot of reading -- maybe too much.  I was all set to go with 3wpg on each of them and then I found out that wpg doesn't work for small tanks like mine. I've been trying to figure out exactly what does work, but I've not been able to nail down a definitive answer.

Somewhere I read something that indicated that one 36w over the 12 gallon would give me high-ish light, while 2 18ws wouldn't. I'm not sure why this would be true, but I'll buy it. Then I was over at Rex's Guide to Planted Tanks where Rex said it was better to use "Lux per square inch." That makes sense to me, too, but I don't know how to find out how many "Lux"es there are to a given CF bulb. Rex says he uses information about the bulbs from A & H Supply, but I don't see where that information is given on the site.

To further confuse things, I looked up a definition for _lux_, and found that 1 lux=1 lumen/square meter. The A&H site says this about _lumens_: "A lumen is essentially a measurement of brightness to the human eye and is therefore very heavily weighted to the middle wavelengths of light that the human eye responds to most readily. As such, this measurement is not very helpful for aquarium applications since the middle wavelengths are the least important to aquarium inhabitants. To focus on lumens can be very misleading. For instance, the 55W 5000K bulb in the chart above has 4200 lumens while the 55W 6700K bulb has 4600 lumens. Yet these bulbs have the same total light output. The 6700K bulb simply has a little more of its output in the middle wavelengths."

So... I'm really confused. I would imagine there are different ideas among the members here as to how much light to use on my little tanks. I want to be able to grow as many different plants as possible, including the "high light" ones if possible. I know that it would be easier to figure it all if I had bigger aquariums, but I don't have room for anything any bigger.


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## Left C (Jun 14, 2005)

Maybe this lighting calculator will help some. I know it's confusing. http://www.fitchfamily.com/lighting.html

It's based on a study of Amano's tanks.

It says: 52w for a 6g and 71w for a 12g.

I have two small tanks. I have a 10g with the 2x40w dual daylight Orbit and a 15g with a 96w 6700K quad Aqualight. With a good CO2 supply and ferts, they really work well. Before I got the Orbit, I had a 2x28w Hamilton on the 10g for years.


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## Deni (Jul 6, 2005)

Wow! I'm going to have to rethink that 6gal. I don't think there's any way to fit 52w on it. Maybe I'll go for low-light there. But the 71w for the 10g is doable.

Thanks much for that link. One of the few pages I hadn't found yet.


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## Gomer (Feb 2, 2004)

While the smaller tanks do indeed need more lighting, they don't need THAT much.

For the small tanks, I find using the 13watt AHsupply kits work nicely. I have 2x13 (the delux kit...the reflectors make a big difference) on a 5g cube and it works like a charm.


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## Blazerfrs (Feb 3, 2006)

There's been some work on the WPG threory over at www.aquariumadvice.com, they're working to more of a lumens/square in based system, taking into account the lumens/watt output of different lighting systems.

it's not 100% yet (work in progress) but I'm impressed:
http://www.aquariumadvice.com/viewtopic.php?t=72491


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## Deni (Jul 6, 2005)

Very interesting. Thank you for bringing that over.


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## acbaldwin (Nov 3, 2005)

Why do smaller tanks need more light? Light intensity decreases exponentially the farther you get from the source, so shouldn't a tank that is shallow require much less wattage? I definitely am lost on this.


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## BryceM (Nov 6, 2005)

acbaldwin said:


> Why do smaller tanks need more light? Light intensity decreases exponentially the farther you get from the source, so shouldn't a tank that is shallow require much less wattage? I definitely am lost on this.


This is a common point of confusion. If you consider the light to originate from a point source (like the sun) the intensity diminishes quickly. If you double the distance from the source, the intensity drops off by a factor of 4. Most aquariums though are lit from a long light bulb or a series of bulbs that occupy most of the surface area of the tank. Light still radiates outward in a straight line, but there is a lot of overlap from the neigbor portion of the bulb.

.................X..........................x.......x.......x......x
...............XXX.......................xxx....xxx....xxx....xxx
..............X.X.X.....................x.x.x..x.x.x..x.x.x..x.x.x
.............X..X..X...................x..x..xx..x..xx..x..xx..x..x
............X...X...X.................x...x.x..x.x.x..x.x.x..x.x...x

I can't get this to look right without making an image (& I'm too lazy) but you get the idea.

Good quality reflectors will also direct the light downward, further reducing the tendency of the light to act as a point source. Distance from the bulb is still important, but it is mitigated by these factors.

Smaller tanks don't actually need more light. They need the same amount of light as any other size tank. The problem comes from our contrived (and very poor) unit of WPG. Gallons are measures of volume, and are 3-dimensional (231 cubic inches). Light is measured in intensity per area, a 2-dimensional unit. Small tanks have lots of surface area while large tanks have lots of volume (to simplify). Small tanks therefore need a higher WPG to get the same intensity of light as in big tank.

Elephants loose very little body heat compared to mice for the same reason. The ratio of body surface area to volume is enormous for a mouse, but small for an elephant. That's why their metabolism has to be so much higher. It's the same thing. I'm still working out how many WPG elephants and mice need so I've got to go.................. lots of math to do.


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## hoppycalif (Apr 7, 2005)

My 29 gallon tank is growing plants pretty well right now with 51 watts of NO bulbs, with a relatively poor reflector. I will soon be installing 110 watts of PC with very good reflectors (AHS kits). I suspect the biggest difference I will see is more rapid growth, assuming I can keep up with the increased demand for nutrients. So, I would say there is a pretty broad range of suitable light intensities that will work for most plants, and I noticed in those graphs that Amano used from 80 to 480 watts for a 80 gallon tank, so that is consistent.


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## acbaldwin (Nov 3, 2005)

Thank you for the explanation guaiac boy. Do you know of any supporting evidence of this? Not to question you, but I am interested in seeing intensity readings and comparisons b/t different tank volumes and heights. Thanks for your help!


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## BryceM (Nov 6, 2005)

The only supporting evidence I have is just the math & physics behind how light behaves once it leaves a source. It's available in any good physics book. To pin down exact light levels and to make comparisons between one setup and another you'd really need a light meter. Submersible ones are a little $$ and for what we do a bit of overkill.

I'd just go with this: With quality reflectors and good quality fluorescent lighting in the appropriate spectrum (say 5,000K to 10,000K) a normal rectangular tank in the 40-60 gallon range can be considered high light with 3 or more watts/gallon. If you have a really big tank 2 wpg produces the same intensity of light on the plants. If you have a small tank you might need 4 or even 5 watts/gallon to get the same light intensity. If your lights are far from the surface of the water you'll need more. If you have a tall tank you might need more (but not much). If your light fixture is above a very thick and dirty sheet of glass you'll need more, etc. etc.

The real proof is in the pudding. Watch the plants. If the tops of the didiplis aren't red you either need more iron or more light.


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## HydroBot (Apr 26, 2006)

I think you'll find that 1 lumen per sq. inch of tank per inch of depth is a pretty reliable rule of thumb. For instance a standard 10 gallon tank has a depth of 12 inches, figure 12 lumens per square in or 20X10X12=2400 lumens. a 36 watt power compact will yield 2200 to 2800 lumens depending on its temp/color. At ten gallons this is 3.6 wpg wich might be considered high wpg for a larger tank but should be just about right for our leader 10. Also with smaller tanks come smaller bulbs, the efficiancy of fluorescent lights tend increase with larger higher watage tubes from a lumen per watt point of view.


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