# Why do sm tanks need more light?



## Dryn (Sep 6, 2007)

I am researching light theories and I was wondering if anyone knew exactly why shallow tanks needed more light per gallon than deeper tanks? Wouldn't it be the opposite?

I was thinking that shallow tanks had less density than deeper tanks, meaning that the water particles were farther apart, letting the light photons "bounce" around more, releasing them through the glass. Deeper tanks have denser water and more compact water particles, keeping the light photons from "bouncing" around as much - slowing them down of course, but keeping them in the water column longer nonetheless.


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## davemonkey (Mar 29, 2008)

Watts per gallon is a rather vague description of the amount of light in a tank. Most people refer to it because it is a good guess and easy to describe to the average hobbiest.

Lets say you have "plantA" groing in the foreground of a 10 gal tank with 30 watts of light (3 WPG). If you want the same "plantA" to grow in a taller 15 gal tank (assume it has the same dimensions, only taller) you would , in theory, need more total watts to get the same light intensity to the plant since it is further from the light source.

So, you put 40 watts on the 15 gal. Now you have more overall watts, but only 2.67 wpg, (less than your 10 gal).

Now, while you have similar intensity at the foreground plants in both, your 15 gal is going to have higher intensity at the top of the tank because you have more light at the surface.

In the 10 gal, you have 30 watts of light at the surface, while in the 15 gal, you have 40 watts at the surface. 

Confused yet? 

Something else to consider is that you can have 7 WPG and still have low light. HOW?! Let's say you have a 1 gallon jar with plants in it. You have a 7 watt spiral bulb over the jar (7 WPG). Well, 7 watts is simply not much light, so it's still low light.

Larger tanks, in general, will need more light overall, but fewer WPG.

-Dave


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## nfrank (Jan 29, 2005)

Rather than re-invent the wheel, take a look at this thred from last year:

why-small-tanks-need-more-watts​
Have fun reading.
--N


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## davemonkey (Mar 29, 2008)

WOW! I only read the first 5 posts, but that is a spectacular explanation. Thanks for linking that Neil.

-Dave


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## nfrank (Jan 29, 2005)

you need to read more of that thred 

Here is some related information compiled by Eric Olson in 1998.
Lighting Levels, although not an explanation, per se.


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## Dryn (Sep 6, 2007)

Thank you all. I definately understand it better now.


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## davemonkey (Mar 29, 2008)

HAHA! Now I have homework!


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