watts per gallon.

[oozy]

ok heres the thing that makes no sence to me, watts per gallon, many people say a certian # of watts are required to keep certian critters. Well, i am sure if you looked at the levels in your tank (hight wise). the intensity at the top would be much higher than the itensity at the bottom. I realise that this watts per gallon would be around the overall average, so my problem is that people told me i could not keep sps corals because my watts per gallon is not high enough. So i went out and got a frag of sps anyway, and now it has more color than it did in the store. So now as i am researching clams. My experience is that if you keep a critter that needs high lighting, keep it close to the lights.
....im done rambling...(im in computer class trying to look busy)
-Oozy-

 

[kris walker]

Hi Oozy,
It seems to me also that the watts per gallon scale is an average to cover all typical tank dimensions. I haven't had many corals yet, but I am willing to bet that the average is justified and important. It makes sense to me since a 40W bulb over a 55 gallon has to spread the entire 40W over the tank bottom (or on lr near the top). Now take the same 40W bulb and focus it on the bottom of a 20 gal tank, and you have increased the intensity of light because the 20gal bottom has a smaller surface area.
I am glad your sps are doing well. How long have you had them so far? I'm sure you know that coral reaction times to changes in environment are very slow, so if you just added the sps to your tank last week, they might not be as healthy as you think. Once tissue recession begins, it can be hard to stop.
sam

 

[javajoe]

I have been wondering about htis myself-- what lighting are you using? and also, how long has the coral been doing ok?

 

[sonny]

The watts per gallon rule is just a guideline. In order to have your whole tank have acceptable lighting to support animals, the rule of thumb is one metal halide per 2 feet of tank (up to about 2 feet wide). You could have a 1000 gallon tank with only one MH lamp over it, and keep corals just fine, as long as they are under the lights. Even if it's a 1000 watt lamp, that's still only 1 watt per gallon, but under the lamp, things would be fine. If you want to keep corals all around the tank, then you have to have about 1 MH lamp per 2 by 2 foot section. The Watts per gallon rule will give you something to go by. The wattage of the MH's depends on the depth of the tank, and of course what animals you're keeping. The deeper the tank, the higher the wattage you need. Fluorescent is not as easy to gauge, but you can bank on having 4-8 tubes over every 2 by 2 foot section of tank. The watts per gallon rule does help when deciding that. Overall, I don't think you can really put too much light over your tank, as long as you're reasonable... don't put 400 watt MH's over a 20 gallon tank. The tank size limits you as to how many Fluorescent tubes you can put over it.
Sonny
Sonny

 

[up north hermit]

Well said Sonny.
Actually the WPG rule basically came about during the days when most Saltwater tanks consisted of fish only and a few hearty inverts. It is absolutely worthless for reefs.
When considering lighting for reefs, and by reefs I mean mostly corals if not all corals, one takes into consideration the dimensions of the tank (mostly depth) and the lighting needs of the specimans to be housed. The intensity is what matters, NOT the wattage.
So if you are lighting a reef only tank, forget the WPG "rule". It simply doesn't apply, IMHO.
My humble .02
Hermit

 

[oozy]

sorry i should have stated what i have....
a 20g with 2 55w pc's (6500&actinic)
ive had the sps for about a month, when i got it it was kind of stone grey, but now has neon green at the tips and large horozontal areas. ive seen little tissue ression before the neon appeared, now the ression has stopped, and i am waiting to see what happens next.

 

[sonny]

For smaller tanks, I do think that PC lights are the best bet. You could even add one more light if there's room over the tank. Metal halide is not practical on a small tank.
Sonny

 

[kris walker]

Hi Hermit,
What is the difference between intensity and wattage? I thought they were the same.
sam

 

[dhe420]

The difference between the 2 is large. Wattage is the amount of energy used to power the light. While intensity is how strong of a light is emitted. A 500 gallon tank with 3 400 watt metal halides is just about 2 watts per gallon. If the tank is 24 inches deep and 6 feet long than you have exactly what is standard. I MH for every 2 feet, and 400 watt 4 24 inches in depth. 18 inches in depth requires 250's, and 12 inches in depth requires a 175MH. Thw watts used are almost completly irrelevent. If someone could design a metal halide bulb that only uses 20 watts but emits 400 watts of lighting or equivalent, wpg would have absolutly no bearing oin the situation at all. It can be very difficlut for "newbies" to understand, especially since lighting is the most highly debated part of reefing. I hope this clears up some misconceptions about lighting. Later..... Dave

 

[dburr]

All we need to do is keep putin down the WPG rule and maybe some day it will die!!

 

[kris walker]

Okay. I disagre and I agree. I just pulled out my physics book, here is what it says about intensity/wattage of light:
Intensity is the power emmitted (watts) per unit surface area that the light is shining on.
For fluorescent bulbs that span the length of your tank, going deeper in your tank *does not* increase the surface area the light is shined on (although some light that would normally shine at the bottom is lost through the front and back glass/acryllic panels). So your intensity stays roughly constant from top to bottom.
For MH bulbs, going deeper in your tank *does* increase the surface area your light is shined on because MH bulbs are like point sources of light. So for MH bulbs, intensity goes down as you go deeper. Dave, this is no doubt why you need to have higher watt MH bulbs for those deeper depths.
Now you probably all know that the red end of the spectrum is absorbed quickly as the light shines into the oceans (this is why there are actinic bulbs). So, yes, the intensity does decrease a tiny bit as it goes through the top of the tank water, but going through 24 in vs 18 in of water...there is basically no signif. difference.
So the output wattage of a bulb and its intensity are not exactly the same, but they are proportional to each other. And intensity is directly related to surface area, as stated in my first post. So strictly speaking, the WPG rule does make sense for flo. bulbs. But it does not make sense for MH bulbs, because intensity is what matters for corals and this scales with depth for MH. This is probably why there is a range of values for the watts per gallon rule.
sam

 

[nm reef]

Hey....I also dislike the WPG rule........and like Sammy said...we could throw standard lamps like the ones in your house sockets(some are rather high wattage) and come up with tons of wattage.....but par and spectrum are what we look for........for example....I run 2x65 watts of CSL PC lamps(combination)..2x65 watt CSL PC lamps(10000k)..and 2x110 watt VHO)actinic blue 03)...thats 8.73 watts per gal......but for me it is a good amount of intensity and spectrum...I keep an assortment of LPS polyps and leathers...all are doing well and would probably die under equal "wattage" of standard house lights

 

[kris walker]

Sammy, the "Watts" reported on a bulb may be the watts input into making the bulb shine OR the watts output by the bulb. I personally don't know. But regardless of this, *for the same type of bulb*, the one with the higher wattage IS brighter.
Also, Sammy and NM Reef, what does P.A.R. stand for?
Finally, if the watts labeled on a bulb are the input watts, then yes, there is a big difference between the energy output by a flo. bulb and the ones output by say a MH bulb. Flo. bulbs are much more efficient than MH bulbs as they do not generate as much heat for a given input wattage. But if the watt labels are for output wattage, then a 100 W flo puts out just as much energy as a 100 W MH. It cannot be merely assumed that the label on a bulb is the input wattage.
NM Reef, I agree, having 100 W of standard white light bulbs (say from your house lamps) is not the same as having that 100 W contained at certain key points of the spectrum.
BTW, I don't like the watts per gallon rule either. So if you want to let it die, lets kill it now!
sam

 

[wally]

In physics class we learned about the transverse-square law of light. (bear with me its been 15 years)What that means is that everytime you double the distance from the light source to the item the light is shinning on you get 1/4 of the light. That is because light expands as you get farther away. So there is a difference between the light at the bottom of an 18" tank and the light at the bottom of a 24" tank. BUT we counteract that by stacking rocks in our tanks. If you place something on a rock that is 4 inches below the surface it doesn't matter if its in a 12" tank or a 36" tank its still only 4 inches below the surface. You can also get away with using less light by placing things higher up.
Light output is measured in lumens not wattage. Watts is the amount of energy used to power the bulb. High effeciancy bulbs like flourescent use less electricty to produce the same lumens as an incandescent bulb. Thats why the little screw-in bulbs that are 15 watts can say on the package that they have the same light output as a 75watt incandescent light bulb. The bulbs with the highest lumens to wattage used in the hobby are metal halide bulbs. A 150 Watt Metal Halide will give you far more lumens than a 150 watt household bulb.
I hope this clears things up a bit rather than confuse people even more.

 

[kris walker]

Lumens and Watts measure the same thing, and that is the energy per unit area, or energy density. And a brighter light for a given area and spectrum has a higher energy density.
1 lumen = 1.5E-3 Watts
1 Watt = 668 lumens
There is no standard for measuring light output. Which scale is used to measure it depends on which community you talk to.
Wally, your Watts vs lumens explanation and example makes total sense to me. If you are 100% sure that the term "watts" as labeled on bulbs refers to the *input* watts, then I'll hang my hat on that. Thank you for clearing that up for me.
But regardless of what you call it, intensity (lumens/area) does not decrease very significantly between deep and shallow parts of the tank for flo. bulbs that roughly span your entire tank length. The source of light is cylindrical, not a point source as your explanation implies. For MH bulbs, I agree with you entirely.
So let me ammend my main point. A generic *lumens* per gallon rule is not a bad one to work with for flor. bulbs. However, it is not good for bulbs that resemble point sources of light (like MH).
sam

 

[junkf15]

I agree watts per gal is not useful, but we need a replacement before it will just die. Let me see if I can put my degree in Physics to some use...
To answer the question about PAR; it is Photosynthetic Available Radiation; also know as PAL (photosynthetic available light). It is the spectrum of light that our critters need to survive, everything else is wasted energy! If we are going to use a rule of thumb for lighting, shouldn't this be part of that standard? I would propose lumens/sqft. That is the unit of interest. It is an easy linear conversion of Watts to Lumens, so Watts/sqft would be equally useful in relative terms, however it would not be as descriptive. I also propose we further refine that to lumens of PAR per sqft. This would be an easy conversion for those of us that use single spectrum bulbs (MH, Antic 03) but it would require those using multi spectrum bulbs (NO, PC) to subtract out the lumens of output light that is outside the PAR spectrum. LP/SF (Lumens of PAR per SqFt) then would be the unit that would be the most descriptive and most useful, and could be used universally between all types of bulbs!
Case in point...
This would be the most useful way for us to describe the inefficiency of the afore mentioned house bulbs. It could easily be shown that while they may be 400 watts and therefore 2.67x10^5 Lumens, very little of those lumens are in the PAR spectrum. If we assume 5% of it's light is PAR, it would only produce 3.34x10^3 LP/SF. Conversely, a single spectrum bulb, like say the 12000K MH bulb, would have 400 Watts, also have 2.67x10^5 Lumens, but they would all be in the PAR spectrum! Over a 2x2 section of a tank, that bulb would produce 6.68x10^4 LP/SF!!
This would explain why we need 1 MH bulb per 2x2 section of tank, because as the light is spread over a large area, that value of LP/SF would go down. It is also a useful number to describe why multiple VHO bulbs are required over a 4 ft. tank. Because they are 4 ft' long, you need one bulb per 6 in. width of your tank so each bulb is still only being spread over a 4 sqft area.
This now can explain what several posts have alluded to. We can have different values of LP/SF in different areas of the tank!! Therefore a coral at the same depth directly under a single MH light is equally happy in a 55gal tank or a 200 gal tank, because the LP/SF he is getting are the same in either case.
Now if we assume that the previous post is correct and agree that the difference between 18 in. and 24 in. deep tanks are not significant, we can once an for all KILL the watt per gal rule, and have something useful to replace it with!
The only question that remains is the exact spectrum width of PAR. My degree in Physics doesn’t help here. Would a biologist out there please clear up for me the exact spectrum of PAR. I think it is centered around the 420 nm region, which is the antic blue 03 peak spectrum, but I don't know how wide the frequency range is.
I hope this makes sense. Please feel free to tell me if you think I am full of it, but if so, please include why you think that.

 

[up north hermit]

Honey, I have a headache.
LOL
Hermit

 

[sonny]

Originally posted by Surfin Sam:
<STRONG>
There is no standard for measuring light output. Which scale is used to measure it depends on which community you talk to.
sam</STRONG>
Not true. Footcandles is a measurement of light used in photography and other areas like television and film production. It does not measure the spectrum, only the level of light. 1 footcandle is the amount of light from 1 candle at a distance of 1 foot. You can buy light meters for use in aquariums that measure footcandles.
All light drops off the farther you move from the source, no matter what the source is. Fluorescent lights are not useful in a tank over 30 inches tall, because they will not penetrate that deep.
Sonny

 

[twoods71]

The wattage rating on a device is the amount of power the device will consume.
W = V * I
A TV consumes more power then a NO bulb but no one puts a TV over there tank.
The wattage rating of a bulb means nothing.
A 40 watt bulb could potentially put out more light then an 80 watt bulb of the same size depending on how efficient the bulbs are at consuming power.
Many appliances today consume less energy then they did 5 years ago but still do the exact same thing.

 

[kris walker]

Hi Junk,
That was comprehensive! I'll have to read it again before I understand everything you said. But from reading it once, and from learning what PAR stands for, I agree that LP/SF seems like a good replacement.
Hi Sonny,
You probably would have liked my two statements you quoted better if it read "what standard is used to describe light output depends on what community you talk to." Light power is measured in Watts in some communities. Thus my confusion.
I don't think we have the room to debate the whole energy-vs-distance issue in tanks for light. If that is your opinion, then fine. In my opinion, for *a flo. bulb that spans the entire tank length*, most of the light that is received at say 2 in depth (near top) is also received at 30 in depth at the bottom. This is due to refraction of light into the water and reflection of light off the tank walls. Very little escapes through the tank front and back.
Hi T. Woods,
You aren't by chance Tiger Woods, the golfer, are you? I had to ask! I understand that the wattage rating on a device is always the amount of power the device will consume. However, I wasn't sure if this carried over into bulbs. In the past when I read labels on flo. bulbs, I was uncertain as to whether the label was reporting output or input Watts. From the above posts, thanks to all those who posted, I now consider then strictly input watts. But as I said above, *for the same type of bulb [and therefore same percent of input Watts lost to inefficiency]*, the one with the higher wattage is brighter, and therefore Watts is not useless when considering bulbs of one particular type (e.g. deciding between an 80 Watt VHO or 20 Watt normal flo. of the same length and manufacturer).
This website and community is truly a great resource, especially if you are a student like me with too much time on your hands! Thanks for all your posts and your patience.
sam