Llama

I was having a think about this, and I don't think "Watts per Gallon" is a good measure of light or anything. I shall explain my point and if someone could explain to me whether I'm right or not (and why) I'd be very gratful.

As I know from physics, water has a refractive index of 1.333 ; so light moves at a slower speed than in air. So assuming that I have a 'shallow' 20 gallon tank with a wide surface area, serviced by a 40 W bulb. Therefore I have 2 W/gallon of light.

Now assume I have a hex tank, with a smaller surface area and which is deeper (or a tank for angel-fish or whatever), again its 20 gallon and there's a 40W bulb. Therefore I have 2 W/gallon of light.

However due to the refractive index of water, the bottom of the deeper tank will be darker than the bottom of the shallow tank - even if they are both 2W/gallon.

Am I right in my assumptions? Is there an alternate way I can measure how much light my plants are getting - taking into consideration the depth?

capekate

good morning,
as I know nothing about physics.. after what I have read about lighting and wattages, I believe you are right. The deeper the tank is going to make the difference.

sirdarksol

You are definitely correct, Llama. This is something that at least a few Fishloreans have been contemplating for quite awhile.
WPG is much like "inches per gallon." It works well in a standard tank, which has a certain width-to-depth ratio, and with standard fluorescent setup.

Part of the reason for this is that, like any fish-stocking guideline, a true rule for amount of light in a tank is going to be more of a calculus equation, involving depth of the tank, how reflective the reflector is, how many surface plants the aquarium has, color and reflective nature of the substrate, using lumens instead of watts, the amount of ambient light in the room, whether the tank is made of acrylic or glass, etc...
Like saying "you should have roughly one inch of typical aquarium fish per gallon of water," "WPG" is easier for someone new to plants to digest. It's a guideline that doesn't always work, and it can be stretched and broken in some cases.

mathas

The best method I know of to measure light is to use a PAR meter (or quantum meter) to measure lighting intensity at a specified depth. Since most people aren't really interested enough to spend $200+ on a meter just to measure lighting intensity, they go with the WPG guideline.

If you're curious, there's a comparison article on two common models found here.

Llama

Allright fair enough, good to see my physics hasn't gone rusty

Thanks for the explanation

MrWaxhead

Yes you are correct but pretty much anything under 24 inches WPG will work pretty loosely, if you are worried about light penetrating the water certain types of bulbs work better then others, I believe the order from a test I read about years ago was. Metal Halide, PC, then tubes like T5HO T5 T8 T12 etc. I saw a chart years ago that tested light levels at depth and halides were far superior for this, I also know a fellow that does alot of plant work for a few local stores where I live and he was part of a study were they took the same species of plants that all weighed the same amount planted them in tanks with similar settings ie size and fish in them etc, and then weighed the plants after a set period of time and the plant weight also came out in the same order with Halides being the heaviest after the test and slowly weighed less from each type of light. It was not a substantial difference but they did notice this trend over numerous test batches as they ran this test on a few occasions to rule out a fluke.

Also each type of bulb changes the WPG formula as WPG was based of T12 bulbs, and other styles of bulbs have a much better Watt to lumen value.

Here is a loose rating based off Lumen to watt ratings if you have other then T12 bulbs.
T12 HO x 0.76
T12 VHO x 0.56
T8 x 1.35
T5 x 1.37
T5 HO x 1.22
Spiral compact x 0.86
PC x x 1.04
Metal Halide x 1.37

Basically if you had a 50 gallon tank with 75W metal halides the correct WPG rating would 75W / 50 Gallons for 1.5 WPG x 1.37 for a true 2.055 WPG.

Or if you had a 20 gallon tank with 2 23W spiral compacts you would have the following 2 x 23 = 46 / 20 for 2.3 WPG x .86 for a true 1.978 WPG.

And even though both tanks would have roughly 2 wpg the tank with the halides would do a better job of lighting the depths of the tank making its 2 WPG more effecient. Granted neither of these tanks would be very deep so it would not be a huge difference but there would be a difference none the less. Where this would make a vast difference is on a tank that was say 30 inches deep etc, then the diffence would be substantial as the halides would do a much better job of lighting the lower portions of the tank.

Llama

Well that made a pretty interesting read, thanks

navyscuba

So let me see if I got this right if my 100 gal tank that is about 22 deep and 19 wide I need about 125 or more watts of light?

mathas

In general that would be a good figure to shoot for, but you might also need more or even be able to get by with less. How much lighting you "need" also has a lot to do with what type of lighting it is, what plants you plan to have, and what you are willing to do for carbon and fertilization.

navyscuba

Ok. Cool