Lighting question - watts per gallon rule?

I have seen a few indications of light strength for aquaria using the "watts per gallon" rule.

[FONT=Arial, Helvetica][FONT=Verdana][SIZE=2][FONT=Verdana][SIZE=2][I][FONT=Verdana][SIZE=2]See here: [/SIZE][/FONT][/I][FONT=Verdana][SIZE=2][URL]http://fish.mongabay.com/plant_care.htm[/URL][/SIZE][/FONT][B][FONT=Verdana][SIZE=2]

[/SIZE][/FONT][/B][/SIZE][/FONT][/SIZE][/FONT][/FONT] However this does not take into consideration depth of the aquaria. I have a pico setup and following this thread I fall into the "subdued" category. There is only 14cm depth till my gravel bed. Does this make a difference? I might be being stupid here

 

The WPG rule is not law. It should be Lumen output that should really concern you. But the WPG is a good way to up your lighting strength in bigger tanks.

For instance 100watts of T8 lighting has a lower Lumen output than 100watts of T5 lighting. The T5's are more intense and more powerful than T8's.

LED lighting is different...

But you have an 8l tank right? What is the strength of the LED? if it is an 8watt LED light you effectively have 4watts per gallon of LED light, which is good.

1 gal = 4l.

 

PAR is a better way to measure. Daylight CFL could also work quite well for a small and shallow tank

 

PAR? What's that?

 

http://en.wikipedia.org/wiki/Photosynthetically_active_radiation

You need special metering equipment to measure output, so gets kind of expensive

 

There is a basic mathematic formula to convert lux to par. lux = lumen per sq. m.

But then depth also plays a role, as you will probably lose about 50% of the light output in a 45cm deep tank.

Then you also need to take into account that lumens measure the entire surface output of the bulb so to get all of that into your tank you need extremely good reflectors.

But all of the above is getting too scientific and specific. Its all good and well trying to gain more knowledge and become more precise in our hobby, but why worry when you can actually use the WPG rule effectively? It was never designed in a lab; it became "common" law after many years of hobbyists all over the world coming to certain conclusions after testing and testing.

Generally, you need 2WPG and you can grow most plants. 3WPG for certain light demanding plants. And this is only true for tanks up to 45cm deep. Any more than that and you need High Intensity Discharge (HID) lamps like metal halide.

Lastly, people bought and sold tanks based on the "sale" size. For example a standard 4ft tank is sold as a 65G tank, so for 2WPG you need 130W. But in my 65G tank I can fit about 200l of water, taking into account substrate depth and so on. So if I measured the water column I would be looking at 105W of light to meet the 2WPG rule. But this is not correct. You should use the "sale" size of the tank, because that is what people used when this rule was developed.

It is not scientific and hardly makes any logical sense, but it works.