# 90g "low light"



## kdh (Mar 11, 2006)

I read on another post something like 70w over a 10g would be the equivalent to 2wpg on a 90g, which got me wondering, "how many wpg are necessary to have a low light 90g tank?" 

I know the wpg "rule" doesn't apply with small or large tanks, but I'm not sure what qualifies as large. That ratio seems to indicate to me that you could grow high light plants in a 90g with only 2wpg, which would mean that low light plants could get away with say 0.5wpg.

Does that make sense? Is it true?


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## czado (May 26, 2005)

You should check out this article from Wizzard~Of~Ozz for his lumens/sq in measurement. This attempts to limit the effects of tank size and bulb type, and while lumens isn't the perfect measure of light, it is a whole lot better than watts. But here let's assume standard tanks, average T-12 NO daylight bulbs, and 100% reflection:

70w T12 over a 10g is about 20.6 lumens/sq in, about "very high light."
180w T12 over a 90g is about 12.3 lumens/sq in, "med light."

While wpg breaks on small and large tanks, your examples or a little extreme. More in line is:

55w T12 over a 10gal (5.5 wpg) is about 16.2 lumens/sq in, high light.
240w T12 over a 90gal (2.67 wpg) is about 16.4 lumens/sq in, high light.

(Once you get to the 125gal range and higher, wpg _really_ breaks. Also more efficient bulbs -- CF, T5, T5HO, T8, etc -- will give out more light per watt.)

If you want a low light 90gal, 80w T12, 50-60w T8, or 65w CF will all get to the low light range (~6 lumens/sq in) using this standard.


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## RoseHawke (Jan 4, 2005)

czado said:


> ". . . (Once you get to the 125gal range and higher, wpg _really_ breaks. Also more efficient bulbs -- CF, T5, T5HO, T8, etc -- will give out more light per watt.). . . "


It does? Sorry to hijack the thread a bit, but I'm considering a 125 for my next tank. I'm also considering that I want a low/moderate (at most) set-up having been the hi-tech route and becoming a bit tired of it. What kind of lighting values should I be looking at using for instance, T8 fluorescent?


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## czado (May 26, 2005)

You want between 6-12 lumens/sq in using this standard. 12 lumens/sq in is right around medium light, 6 lumens/sq in is low light.

Assuming the average bulb, a standard 125gal, and 100% reflection, here's the lumens/sq in for some possible configurations (I believe 36" normal output T8 bulbs are rated for 24watts):

```
Eq T12 wpg    Lumens    Lumens/sq in
72w T8         0.89     6559.2      5.06
96w T8         1.19     8745.6      6.75
120w T8        1.48    10932        8.44
144w T8        1.78    13118.4     10.12
168w T8        2.09    15304.8     11.81
192w T8        2.38    17491.2     13.50
216w T8        2.67    19677.6     15.18
```
You want to compare these values to Wizzard~Of~Ozz's database of real-life tanks.

So, 100w-175w T8 is nice low-moderate light and should be what you're looking for. 200w T8 is in that med-high/CO2 light level. Here's a bonus if you used 2x 36" T8 NO and 2x 55w CF. (You would use this Excel/O calc for combining bulb types or calculating theoretical tanks so as not to skew Ozz's database of known tanks.)

```
Eq T12 wpg    Lumens    Lumens/sq in
48w T8 + 110w CF      1.78     13106.8  10.11
```
Hope this helps.


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## RoseHawke (Jan 4, 2005)

Thanks =D> , just what I'm looking for!


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## hoppycalif (Apr 7, 2005)

I suggest not over engineering the lighting. You could use a four light fixture giving 2 watts per gallon, with the option of disconnecting bulbs to reduce it if you need to. If you start with only 1 watt per gallon and need more light it is a lot harder to get it. If you used 4 - 55 watt AHS lights for the 90 gallon tank, you could use .6, 1.2, 1.8 or 2.4 watts per gallon, with unequal distribution, or 1.2 and 2.4 watts per gallon well distributed. I don't think anyone has yet demonstrated that lumens per unit area is a good way to pick lighting.


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## joephys (May 9, 2006)

Sorry I dont' have time to read the article right now, but lumens is bad way to measure light. Lumens is a measure of how bright light looks to the human eye. You can have the same amount of yellow-green light as you do red or blue, but since yellow-green light is easier for the eye to see, its lumen value will be higher. The best way to measure lighting is in the wattage of the light (I am refering to the power of the actual light, not the electrical power used.) I have never seen lights that are labeled in this way.


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## Wizzard~Of~Ozz (Jun 9, 2006)

I can understand trying to keep is simple, and the WPG does just that. However I would say that lighting is not simple. The name of this forum "Science of Lighting" would imply that I am correct.

The 2 major complexities with the WPG rule are actually pretty easy to understand..

1: Lighting choice. a Compact fluorescent bulb is not equal to a T12 per watt, otherwise "energy efficiency" would be a complete lie. To assume so can lead to some major problems with an excess of light.

2: Area. This one you can see easily. Put a 60w incandecent lightbulb in a kitchen that's ~ 12' x 14'. Not that bright in the room. Take that same lightbulb and put it in a 5' x 8' washroom. Much more suited.

While gallons are an easy to use number they are volume and volume can come in many shapes and sizes, Lighting is designed to fill an area with light. Indirectly volume becomes a factor, but that's only because 2/3rds of the formula for volume is area.. Loss through the water has an effect however Plants don't grow at the bottom of the tank (with the exception of carpet plants), They grow towards the light. Insufficent light at the bottom would cause root rot, which is something lumens/sq" does not cover and is not typical if you consider the approriate plants for your light level. 

Lumens are not a perfect figure, however it's in the interest of keeping it simple.. Not everyone will take the time to break down the spectral analysis of a bulb. I've found that using lumens converted from known working tanks (derived by the WPG rule on a 55Gal tank which is a median) with the average lumen rating/watt of known tri-phosphor bulbs with a colour temperature of 5000 - 7300K would yield more accurate results then a typical generalization.

WPG as far as I'm concerned, when it comes to different bulb types, is no more accurate then comparing a corvette and a chevette as eqauls. (They are both cars and both have 4 wheels so they must be, right?)

Just hoping to clarify some of the thought process behind it.


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## czado (May 26, 2005)

Also, the availability of lumen ratings to do the deviation on samples is important. If lumens per unit area is only less-imperfect than wpg, it holding up in surveys and real-tank examples means it has some value. In my mind the question is if it has more value than watts (energy consumption) per gallon. In my mind it does.


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## kdh (Mar 11, 2006)

czado said:


> If you want a low light 90gal, 80w T12, 50-60w T8, or 65w CF will all get to the low light range (~6 lumens/sq in) using this standard.


Were you reading my mind?

Basically $$ is a limiting factor for me so I'm pretty much stuck with looking for the cheapest options out there. Mail order retro-fit kits are out of the question.

One option is shop lights from Home Depot. The two choices are 2x40w T12 or 2x32w T8. According to the ~6 lumens/sq in guideline I should be on the low end of low light (given the poor reflectors) with these options. I'd like to try to sqeeze in another single bulb under the hood but I'm not sure if it would fit. The good think is that the inside of the canopy is painted white so I might get a little more reflection out of that.


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## bpimm (Jun 12, 2006)

kdh said:


> Were you reading my mind?
> 
> Basically $$ is a limiting factor for me so I'm pretty much stuck with looking for the cheapest options out there. Mail order retro-fit kits are out of the question.
> 
> One option is shop lights from Home Depot. The two choices are 2x40w T12 or 2x32w T8. According to the ~6 lumens/sq in guideline I should be on the low end of low light (given the poor reflectors) with these options. I'd like to try to sqeeze in another single bulb under the hood but I'm not sure if it would fit. The good think is that the inside of the canopy is painted white so I might get a little more reflection out of that.


I may be to late on this but I'll through it out anyway. I have a 90 that I'm setting up also, I was shopping at homedepot and they have Electronic ballasts that can drive 2 32W T8 bulbs for $17, and 4 32W T8 bulbs for $19. They also had "Daylight tubes" for $2.50-6.00 each Sockets are $1.00 each.

If you can install these in your hood (if you have a hood) you could have 4 T8's for under $40.

Beware of cheap shop lights, some of them shorten bulb life dramatically.

Brian


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