Oh glorious Light.

One thing that we have all noticed is the WPG rule, embedded in our heads just like the 1 inch per gallon rule. Both have been proven inaccurate at best. This is also the most commonly asked about question in planted aquaria.  I hope this will help with deciding on what the best fixture is for your goal with your tank.

Debunking an old rule

First I would like to address some major flaws (and some addressed by czcz) in the WPG Rule.

• The WPG Rule does NOT take into account tank depth or shape

• The WPG Rule does NOT take into account anything but T12 output

• The WPG Rule fails in small tanks (thanks for pointing this out czcz)

• The WPG Rule fails in large tanks (again, thanks for this)

• The WPG Rule fails to address the output of a bulb

I will explain each item noted above.

1. The depth of the tank has significant differences in the light that can reach through the tank. A gallon can take many shapes, it is fluid. This is why we can purchase a 20Gal XH and a 20Gal Long. Both are 20Gal, but they aren't the same. Should they be treated the same? No.

2. With the advent of brighter fixtures, outputting more light per watt. The WPG Rule cannot take this into account, it isn't flexible.

3. This was mentioned to me, and upon examination and reversing what I will show you later, it holds very true.

4. This is the same problem as above.

5. A bulb is a bulb is a bulb, then why are there so many choices? you have 6500k, actinic, 10000k...the list goes on, What's important? what's not? hopefully this will help.

In depth

Firstly, I am NOT a scientist, I have plants, a tank, some fish.. But what about lights? I looked and looked and kept coming across this WPG Rule. Following it to a tee led to green water, but why? it was only at 120w.. First, the changing of the bulbs.

At first there was 1, you could drive it higher, but it was still the same bulb. A T12 Fluorescent. The T12 (T# where # is the number of eighths of an inch in the bulbs diameter, so 12/8 = 1 1/2 inch diameter) bulbs come in a lot of lengths, but they are all rated at 10W / foot. This worked well for most people for a long time, but then something happened. In the midst of a push to reduce energy consumption new light bulbs and ballasts were born, the first to come off the line was a T8, it was 4 feet long and only consumed 32w (so now 8w / foot), But there is more to this bulb then mere wattage.  We now introduce an efficiency factor. The T8 bulb, while it consumes less power actually outputs more light per watt. Let's compare a readily available bulb

As you can see, the lumen output of the T8 exceeds the T12 by 450 lumens, and it does this at a lower wattage. So what does this mean? well if we look at the difference in the outputs compared to watts, (lumens/watt) we see that a T12 operates at 58.75 lumens per watt. The T8 operates at 87.5 lumens per watt. So to put this into perspective, the T8 outputs 141% of the light that a T12 does. Following the WPG Rule on a 30Gal tank aiming for 4WPG leads to 120Watts of T12 or 7,050 lumens. Now compare to 120w of T8 that the WPG states is equal putting out 9,950 lumens.  Interesting, you have now exceeded your goal by 3000 lumens or 41%.  If we reverse the numbers and see how many watts of T12 lighting it would take to create the same intensity of light we find it's 168.7Watts, or 6 WPG. How can this be? Simple efficiency. Old school rules can't be applied to new school technology.

T5's and Compact Fluorescents (CF) I will only touch on since there is no direct bulb to bulb comparison, however I will state that the average bulb outputs 79.4 lumens per watt (135% of T12) . Interesting that a T8 bulb actually outputs more light then a T5 or Compact Fluorescent bulb. Not really. Their specialty is in long lasting and compact location.. A CF or T5 bulb will burn out before it becomes spectrum dead. The long lasting bulb makes them far more efficient, also the compact design means you can fit more light in the same space, but it would seem the sweet spot for efficiency (only by a margin) is T8. 

So what have we seen thus far? T8's are 41% brighter per watt then a T12, and a T5 closely behind the T8 with 35% gains. Now T8 looks like the best choice right? Yes and No.. To explain: a T8 bulb has an effective lifespan of 6-9 months before the phosphors that are responsible for making the spectrum required by plants breaks down. This leaves the bulb producing erratic light for the plants. You will NOT see this, it effects only the spectrums that the plants use to photosynthesize. When they become erratic they can produce excessive blues, or more typically reds, this can lead to algae and other unwanted growth. Effectively you should replace your light bulbs after 6 months, but no later then 9.. Compact fluorescent  on the other hand will actually die before this happens, so there is never the threat of erratic lighting.. Also the bulbs tend to last 1 to 1 1/2 years. For cost, T8's are still where it's at, the bulbs can be obtained cheaply.  Perhaps time will bring CF lighting down in price.

Breakdown of bulbs efficiency:

Creating a new rule based on Constants

Firstly, there are 2 things that plants need. 1 is Spectrum, they need a full spectrum of light. The closest achieved in artificial fluorescent lighting is tri-phosphor using 1 phosphor for red, 1 phosphor for blue and a rare earth phosphor for green, the common misconception is that plants require only red and blue, this is true for algae, but not plants. Without green light they will not get a rich green colour on their leaves. 2 is intensity, this is the major failure of the WPG rule and applies to the latter 4 points on shortcomings.

So how can we make this useful? first we'll take a tried tested and true aquarium that works with the WPG rule. The 55 Gal. But how can we break this down into useful measure? well we can start by converting different light levels to lumens instead of watts. We'll use a base of 59 lumens per watt.

With that knowledge, how can we apply this to different tanks? well we know the dimensions of the tank. Working out the lumens / Sq inch of surface area is a good start. (48inch x 12inch)

How is this information useful?

You have a 10Gal tank, you want to hit Very high light. That would require 22.43 lumens / Sq. inch, there are 200 sq inches in a 10 gal (20x10) so 22.43 x 200 = 4486 Lumens or 76 watts of T12 NO light (4486 / 58.9 Lumens/watt) or, if you are going the T8 route. (4486 /82.8 Lumens/watt) = 54 watts of T8 lighting.

We will have to disregard the depth difference for now since we have "tried and true" information that shows height has little to no impact under 24" I will update this if I find out otherwise..

So, getting the light level you want from the above table (the one with blue text) and multiply it by the surface area in your aquarium. Divide that number by the Lumens/Watt of the bulb type you've chosen (table with red text) And that is the wattage I would recommend.

Also, if you want to get it really close, you can research different tri-phosphor bulbs (full spectrum) and get their lumen output and use this directly.

This article assumes Average Reflectors ,it is based on real world numbers.

If you would like to contribute you Current tank setup to add to some building data, please visit here.

Reference: (Courtesy of czcz)

The Bulb data was taken from Ivo Busko's data on aquabotanic