WPG question

[vmahaffe]

I have heard everything listed as WPG, like low light plants 1-2 WPG and high 3-4, but when does depth come into play? A 75 gallon tank has basically the same depth as a 125 gallon, but obviously the WPG is different. Is there a standard based upon the tank depth? Maybe a WPI(watts per inch)?
Thanks.

[phanmc]

The wpg "rule" is by no means a scientific formula, it is simply a guideline that was established by taking observations from many planted tanks to see what amount of lights work for the more commonly found standard size tanks. People have found that the wpg rule works well for standard dimension tanks that are between 20g-100g. The wpg rule also was established with regular fluorescent lights.

The wpg rule doesn't work so well if your tank is smaller than 20g, larger than 100g, has unusual dimensions like tall hex tanks, or uses lights that are not standard fluorescents.

[Nolapete]

It's more of depth rather than larger than 100g that creates the problem. Anything more than 18" deep is going to require more wpg than the general wpg rule implies. It takes a little common sense and experimentation. Some people may have excellent results with 2 wpg on a 14" deep tank whereas some others may struggle with 4 wpg on the very same tank. There are a lot more factors involved than just plugging in gallons * wpg to have success.

[amosf]

The wpg rule is about as useful as the inch per gallon rule.

In other words, not much at all.

It's a rough hint at best...

[phanmc]

The wpg rule is actually quite useful if you know it's limitations.

If your tank fall under these parameters:

between 20-100g
approximately standard dimensions
Using NO or HO fluorescent lighting

It works very well, and I will hazard to guess that many people fall under these parameters.

[Rex Grigg]

It's more of depth rather than larger than 100g that creates the problem. Anything more than 18" deep is going to require more wpg than the general wpg rule implies.

This statement reminds me very strongly of what comes out of the south end of a north bound bull.

The standard large tank in the US is the 55 gallon. And it's 21" deep. With decent reflectors you can grow most plants at 2 wpg.

Can you backup your statement Nolapete?

And yes I am on a crusade to stop the ignorance.

[snickle]

I take the wpg rule as a suggestion, nothing more. But it is as good guesstamite. The Lux per Gallon rule, I have not figure out how to calculate considering it involves a distance factor.

Depth is a factor. I have a 10G which is 12" deep with a 13W bulb. Riccia mounted to slate grows like a banshee.

My 90G is 24" deep with 210W of light and the Riccia in their is healthy but very slow growing. I also have trouble with all ground covers, taller plants thrive.

[Nolapete]

Use a light meter at the bottom of an 18" tank and then at the bottom of a 12" tank with the same wpg and tell me how much ignorance there is in my statement then.

I will concur that 18" was probably a bad choice for an example, but it was merely that an example. It was in response to his statement that a 75 and a 125 are close to the same depth and how does the wpg rule come into play in that situation.

Let me clarify my point and say that the depth of the tank IS a factor that you need to consider in addition to the wpg rule/suggestion.