With so many new technologies and different lighting types watts per gallon just doesn't work any more. Here is a new method that works for all size tanks and lets you compare lighting technologies to find the one that will work for you.

Introduction

I'd like to start this article off by an open admission that I am no expert. I have been doing excessive amounts of research in trying to get the most effective lighting for my tank and noticed that with the introduction of new lighting technologies the old watts per gallon guideline is no longer really that valid.

Currently the lighting rules state that for strong lighting you need >2watts light per gallon(US). This assumes that you are american and that your tank is of average size and that you are using standard T8 flourescent lighting. So how does this relate to T5 or Metal Halide lights? How does it work in nano tanks or really large tanks? Truth, it doesn’t work at all.

Now some terminology, this is pretty basic stuff to anyone that’s done a physics GCSE or remembers their O-levels but its good to be clear.



How bright is a Watt?
For most domestic purposes you buy lights basing the brightness on watts. This is a little like judging the speed of a car based on how much petrol it uses.

This has always worked because the same technology lights follow the same efficiency rating and a 100W is always brighter than a 30W. Now though you may have noticed Energy Saving bulbs have been on sale for a while claiming that an 11W Energy Saver is equal to a 60W regular bulb.

With aquariums its even more complex though because you have T8, T5, T5HO and occasionally the older T12 tubes, then you have to compare that to Mercury Vapour, Metal Halide, HPS and even daylight Energy savers. So instead of concentrating on how much energy you'll be using this guide will show you how much light your plants need and the various methods you can accomplish this.

For different lighting types we can find an efficiency rating of how many lumens produced per watt of electricity. Most technologies become more efficient the more power flows through the light. For this reason given the choice of using 2x 24W tubes or one 48W tube you should use the 48W tube. Not only will it be cheaper but also provide better lighting for your plants using the same energy to run.
You will also find a small variance in Lumens with brand and age of light. Also for fluorescent lighting the quality of the starter will effect the efficiency of the lights.

Keeping the above in mind this table can be used as a guide. Actual Lumens per watt can vary but unless you can find the exact numbers for the lights you are considering assuming the following figures should help you to work out what is required.



Colour Temperature
You may know that brightness isn't the only important factor for your plants. Plants rely on the suns light for photosynthesis and normal incandescent lights are pretty much useless for them no matter how many lumens you provide. For this reason aquarium lights are given a colour temperature, for freshwater lights this should be between 5000K and 10,000K. I'm not going into detail about this here but I would like to point out that Kelvin is almost as useless as wattage. It's the measure of the yellow light spectrum not the spectrum that the plants use so again falls down as you start to compare different lighting technologies.
There is such a thing as Correlated Colour Temperature which is more accurate for fluorescent tubes but not always used. If you want more information please Wiki it, but for now it is enough that we are aware that the K of a light is important but isn't the end of the story.

The Importance of Lumens
The sun is very, very bright. It shines very strongly on the tropical regions where the plants we have in our tanks come from. We measure that brightness in Lux, or lumens per square meter. From this we can recreate the right levels of brightness in our tank.

Light Quality
While the quantity of light in the right colour temperature for the plants is probably the single most important factor the quality of the light will make a big difference to how we perceive the tank.
Even if all the figures work well for you the tank might end up looking horribly dull and flat. Also bear in mind that the fish in the tank will be more effected by the light quality than the brightness. For this reason most people stick to specialised fluorescents or metal halide, High Pressure Sodium or Mercury Vapour designed specifically for aquatic use.

Reflectors
Manufacturers of reflectors claim that using them increases the lighting to the tank by 100%. This is never going to happen. However they are a useful addition to best use the light you have available.
With a closed lid tank that has a clean, flatt gloss white or silver finish inside most of the light is reflected back into the tank without the need for reflectors. Take the lid off and you will lose nearly 50% of the light generated. For open top tanks reflectors are a very good idea, in closed lid tanks they normally only serve to stop you being blinded when you open the lid. Still nice to have but they wont add 100% to the lumens of your tube. The lumens per watt given above assume the light is used in a closed lid tank or in some way directed towards the water surface.
For a tube fitted with a clean, efficient reflector you can multiply the lumens per watt by 1.2, for those in an open top tank without a reflector multiple by 0.7 and for those in an old tank with a darker non-reflective lid pick a number in between.

Putting it all together
Well by now you might be wondering what all this science geek stuff means when you are looking to upgrade your lights. Stay with me because the answers are right here.

Although the LUX level in tropical areas can be 70,000, in the shallow clear waters of a home aquarium we can consider the following as a guideline for tropical plants:
Strong Light : >20,000 Lux
Medium Light : 12,000- 19,999 Lux
Low Light : 6,500 - 11,999 Lux

Next we must work out the square meters of your tanks base. This is simple maths, example tank:
Tank width 1.22m
Tank Depth 0.45m
Tank Height 0.5m
274 Liter capacity
250 Litres of water

Area = 1.22m x 0.45m = 0.55 Square Meters


So now we multiply the LUX by the surface area to get the lumens that the tank needs. I want medium strong light in this tank so I'm going to aim for 33,000Lux. Strong enough to grow almost anything.
0.55 x 33000 = 18,150 Lumens

So how do we get this into the tank?
T12's ?
With lid : 18,150 / 40 = 453.75 Watts
Without lid : 18,150 / 28 = 648 Watts

T8
With Lid : 18,150 / 60 = 302.5 Watts

T5HO's ?
With Lid : 18,150 / 80 = 226 Watts
Without Lid : 18,150 / 56 = 324.1 Watts
With Reflector : 18,150 / 96 = 189 Watts

Metal Halide ?
18,150 / 90 = 201.7 Watts

On the other side of the coin what if I am considering buying the 4x 39W T5HO lummierre from e-bay?
4 x 39 = 156W
156 x 96 = 14,976 Lumens
14,976 Lumens / Surface area of tank (0.5) = 29,952 Lux which would be good for all but the most demanding plants.

The last point to take on board is that water scatters and absorbs light. These figures should work for most tanks around the 50cm tall mark assuming clear water. If your tank is more than 60cm tall you will need to increase the amount of light a little.