Deni

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.

Left C

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.

Deni

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.

Gomer

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.

Blazerfrs

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

Deni

Very interesting. Thank you for bringing that over.

acbaldwin

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.

BryceM

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....xx x....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.

hoppycalif

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.

acbaldwin

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!