How do people come up with this lighting ratio?

[glaekenjz]

If you have a hundred gallon long, that is 18 inches deep, and a 55 gallon tall, that is 24 deep, you can have the same type of lighting, but the watts per gallon would be higher in the 55. My basic understanding of science tells me that light diffuses going down, not laterally. So, it doesn't matter how wide your tank is, just how deep. Am I way off on this one?
I am concerned because I received a few sps frags recently from the LFS. I know that they should only be attempted with Metal Halide, which I don't yet have at the moment, but I am stubborn. And they are doing good so far sitting on the very top of my live rock. They have both grown considerably, almost doubling in a month and a half. But I only have 440 watts of VHO over a 125, that is 18 inches deep. Is their "success" shortlived? Are they eventually going to turn white and die, if I don't get metal halides in time?

 

[ctgretzky9]

watts per gallon is an old standard from years ago, and I am baffled as to why people still use this formula. It is bogus especially with so many different "non-standard" tank depths and sizes, not to mention acrylic custom tanks.
I say if you are having great luck with what you have now, and things are growing and thriving, stay with it. 440 watts in an 18" deep tank seems fine.

 

[bang guy]

It depends on the SPS, not all require intense lighting and not all can live under VHO. Water quality and feeding is probably more important but MH (when you get it) will help IMO.

 

[oceana]

the WPG idea in short was come up with to give a noob a general guide to go by.
with watts per gallon you can very quickly tell someone without going into detail if they have enough light or not. this avoids going into detail about placement.
if you tell me you have 440 watts of VHO i would say no you dont have enough light per gallon to keep any and every sps. but if you said you are putting your coral 2 inches under the surface i would say that you SHOULD be ok for some sps.
if somone says they have 800 watts on a 55 gallon tank you would be safe to give a basic answer yes to just about any coral they talk about and not have to give thought or get into coral placement. its just a general guide without going into detail.
most people have standard size tanks so for a quick answer WPG works well.
other then that its useless.

 

[bang guy]

I agree.
Watts Per Gallon is a very useful number as long as it is understood that it's just a general starting guideline. The experienced reefkeeper can easily judge lighting just based on "feel" given the tank dimentions and the lighting type but the new hobbiest needs some quick equation to get an idea of what's needed. The research should start from there.
I think it gives the new hobbiest a hint that the NO striplight isn't going to be adequate for a reef and that's a good thing. If you create a formula that incorporates tank depth, lighting type, and animal requirements I can guarantee you nobody will use it. It will lose the new hobbiest in its complexity and the experienced hobbiest will already know what their lighting requirements are.

 

[ctgretzky9]

When you say it that way, as a general indication of basic lighting, then I can see how it can be usefull in a quick analysis.
In another way, however, I can see how it can also be a problem with someone like me who has a 60 hex where the depth is twice that of a standard 55. I guess Im speaking more for my own tank experience vs what most people would have. Damn hex.

 

[bang guy]

Originally Posted by ctgretzky9
I can see how it can also be a problem with someone like me who has a 60 hex where the depth is twice that of a standard 55. I guess Im speaking more for my own tank experience vs what most people would have. Damn hex.
Well, while I agree with you, let me put it this way:
for 8wpg you would need 480 watts over your tank. Tell me how you're gonna do that with NO strip lights You're gonna need a 400 watt HQI pendant just to get close. See how it eliminates the less intense lighting schemes?
It's far from perfect but it gets people thinking and that's what matters most.

 

[glaekenjz]

Yeah, I see what everyone is saying. I will get my metal halides soon, but I can't right now. I know that they are better, higher PAR value, lumens, etc. I just wanted to make sure that I wasn't going crazy questioning this rule, that so many people try to endorse.

 

[dburr]

Originally Posted by glaekenjz
If you have a hundred gallon long, that is 18 inches deep, and a 55 gallon tall, that is 24 deep, you can have the same type of lighting, but the watts per gallon would be higher in the 55. My basic understanding of science tells me that light diffuses going down, not laterally. So, it doesn't matter how wide your tank is, just how deep. Am I way off on this one?

I think you got it.
I like to use the 10 G and the 100 G tanks. Both lite with a 175 MH. Each has the same coral and each 3" under the surface. Which coral will get the most light? One is 17.5 WPG and the other is 1.75 WPG. :thinking:

BTW, now you have started the SPS bug. You will be getting MH in the future, I can see it now.