# Wattage and CO2



## MatPat (Mar 22, 2004)

When I started in the planted side of the hobby in late 2003, I seem to remember 2 watts per gallon (2 wpg) being the cut off point for when CO2 was needed in a planted tank...CO2 was not necessary under 2 wpg but would be needed above 2 wpg. Back then, most of my reading on this "theory" was found on the The Krib and people were generally using shop lights with inefficient (by today's standards) T-12 fluorescent lamps and magnetic ballasts. It seems the watt per gallon "rule" was created based on T-12 lamps and magnetic ballasts.

I now see the same recommendations (2wpg) given as a cutoff point for CO2 usage regardless of what type of lighting an individual chooses to use over their tank. Today, most of our lighting is not only more efficient than the T-12 shop lights and magnetic ballasts of the past but it is also much more intense. T-8 lighting, compact fluorescent (cf) lighting and T-5 lighting is now available for use over our aquariums. These systems manage to squeeze out more light per watt than the old T-12 shop lights and most have a better quality reflectors as well.

In the past, I have ran three T-8 shop lights (192 watts or 2.5 wpg) with very poor quality reflectors over a 75g tank and ran into algae problems when CO2 supply was inadequate. Even after dropping back to two T-8 shop lights (128w or 1.7 wpg) I could still create algae issues with less than adequate CO2 supply. A fellow SWOAPE member, NeonRob, uses a TEK fixture with two 54 watt bulbs over his 75g tank. While this is less than 1.5 wpg he still runs into algae problems when his CO2 tank runs dry or he has less than optimal CO2 in his tank.

I understand reflector quality and light output can vary depending on the brand of fixture used but how can we give proper guidance to someone new to the hobby with today's more efficient and intense lighting systems?

Should we try to revise the WPG "Rule" to make it easier for new hobbyists to succeed with a planted tank?

Does an aquarium with 1.5 wpg of AHSupply lighting need to have CO2?

Does an aquarium with 1wpg of T-5 lighting need CO2?

What are your recommendations as far as lighting and CO2 usage are concerned?


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## jeff5614 (Feb 15, 2006)

I definitely think it needs some revision. I, like your friend, have 2x54 T5HO over a 75 gallon and my plants grow like weeds and can't even imagine trying these lights without CO2.


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## MatPat (Mar 22, 2004)

Wow, one response out of 70 views? Ya'll have to have some kind of opinion on the different types of light we use over out tanks 

I see so many new folks starting out with 4 or even 6 T-5 lights over their tanks and in my opinion that is way too much light for most hobbyists. In this thread dooboogoo want to use a 2x55w AHSupply kit over his 29g tank and in this thread, BJRuttenberg is asking for a clearer, more concise guide to help people with lighting their tanks.

Granted, there are a few who can manage veryhigh levels of light, but most of them have been in this side of the hobby for years and don't run all of the lamps at the same time. Light is the primary driver when it comes to plant and algae growth. If someone puts too much light over their tank and ends up with algae issues it can be a real pain to get that fixed. What better way to loose interest in a planted tank than to create an algae farm?


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## Tex Gal (Nov 1, 2007)

I agree with you Matt. I just went fast track for 8 mths now and am loving it. I didn't even think about wpg before then. I have learned so much in the 8 months. Before then I went to the LFS and he called his supplier and I ended up paying 2x's a much as I would have. At least I got decent lights. That was about 4 yrs. ago. But I still didn't get the light/ferts/CO2 thing. All I knew is I needed fert tabs, excell and light. Didn't get the rest.

I'm not sure how you get to any general rule of wpg now however. Lights have gone high tech themselves.... When you consider all the "T" bulbs out there and then add the HO and VOH... But don't forget about the reflectors, none, mylar, sheet metal, aluminum, white paint (pure white or gloss?), etc.. It gets mind boggling. We haven't even mentioned type of bulb yet.... I doubt anyone can get to a general rule unless they qualify what bulbs, the reflectors, the tank depth....

There are so many variables. It gets confusing. I think that's why so many people ask for light help.


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## Left C (Jun 14, 2005)

There would also need to be wpg recommendations concerning the size of the aquariums as well as the different lighting types that are available in today's market.

... like nano, small, medium, large, extra large, giant or something similar.


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## ericpop27 (May 13, 2008)

I think what we basically need is an excel spreadsheet that has the types of lighting on one side and the types of carbon on the other.

T-8, T-12, HO, VHO, CFL, MH, etc.
Flourish Excel, DIY CO2, Pressurized CO2

And then in the middle it could specify which wpg you'd need to have it at.


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## MatPat (Mar 22, 2004)

Tex Gal said:


> I agree with you Matt...I'm not sure how you get to any general rule of wpg now however. Lights have gone high tech themselves.... When you consider all the "T" bulbs out there and then add the HO and VOH... But don't forget about the reflectors, none, mylar, sheet metal, aluminum, white paint (pure white or gloss?), etc.. It gets mind boggling. We haven't even mentioned type of bulb yet.... I doubt anyone can get to a general rule unless they qualify what bulbs, the reflectors, the tank depth....
> 
> There are so many variables. It gets confusing. I think that's why so many people ask for light help.


I would agree with you TexGal. I had very good luck with Toninas and Eriocaulons with about 3wpg of T-8 lighting (overdriven of course) and no reflectors. The light was simply reflected off of the top of the canopy, which was painted gloss white...not the most efficient form of reflector but it worked none the less.

There are so may lights out there today it can be very confusing. Most everything out there today is much more intense than the T-12, magnetic ballast powered shop lights of years ago. Is it safe to say, in your opinion, that 2wpg of AHSupply lighting is the upper limit or should that be a bit lower for your tank? Assuming of course you are using AHSupply lighting.



Left C said:


> There would also need to be wpg recommendations concerning the size of the aquariums as well as the different lighting types that are available in today's market.
> 
> ... like nano, small, medium, large, extra large, giant or something similar.


Indeed. I think the width of the tank also comes into play...you can get very similar results in both a 55g tank and a 75g tank with the same fixture over both so how does this play into the equation?



ericpop27 said:


> I think what we basically need is an excel spreadsheet that has the types of lighting on one side and the types of carbon on the other.
> 
> T-8, T-12, HO, VHO, CFL, MH, etc.
> Flourish Excel, DIY CO2, Pressurized CO2
> ...


 This is a good idea! An Excel spreadsheet or even a chart might be beneficial...something similar to the KH/pH/CO2 charts we used to use. It would get people into the ballpark much better than suggesting 2wpg of light, regardless of their tank size. I do not feel that DIY CO2 is very practical on tanks much over 30g though. I even had algae issues (mostly hair algae) with 2wpg of T-8 light (the typical lights that come with a 10g tank) over a 10g tank. The normal dose of Excel was not sufficient in this case to keep algae away but doubling the dose did cause it to receed.

What would the wpg limit be for Excel or DIY?


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## ericpop27 (May 13, 2008)

As far as the tank sizes go, we can repeat the chart for each range of tanks. Maybe from 1-20 Gals, then from 20-55, 55+ or however it is best to break them up.

You know, it must be right AT 2wpg that people have trouble. Even with standard lighting.


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## Afyounie (Aug 10, 2007)

This is a really good idea. Splitting the spreadsheet into multiple sets is a good idea. 

I know that when you get down to the nano size you need a lot more light than you think you do. I have 80W over my 10 gal, and it is finally growing nicely, but not out of control. I have 45 watts over my 20g and everything grows like weeds. I have pressurized co2 on both.


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## Tex Gal (Nov 1, 2007)

Afyounie said:


> This is a really good idea. Splitting the spreadsheet into multiple sets is a good idea.
> 
> I know that when you get down to the nano size you need a lot more light than you think you do. I have 80W over my 10 gal, and it is finally growing nicely, but not out of control. I have 45 watts over my 20g and everything grows like weeds. I have pressurized co2 on both.


This is a prime example of why we can't say 2wpg, even if we had the same fixture with the same bulbs. I love the spread sheet idea. It just might work.

Where is Hoppy's input?! He did great things with that light meter.....


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## MatPat (Mar 22, 2004)

ericpop27 said:


> As far as the tank sizes go, we can repeat the chart for each range of tanks. Maybe from 1-20 Gals, then from 20-55, 55+ or however it is best to break them up.
> 
> You know, it must be right AT 2wpg that people have trouble. Even with standard lighting.


2wpg has always been the trouble point for me, but I tend to use T-8 lamps on my tanks...I know, I'm "old school" but I do have a CF fixture on one tank  I can only imagine what CF and T-5 lights would do especially for someone new to the hobby who had not grasped the idea of fertilizers and CO2.

I just ran across this thread, a sticky at the top of the forum. It looks like John N. started some nice work on this same topic back in late '06. I guess I need to read the stickies a bit more often 

While John's chart is nice, I would love to see the spreadsheet idea put into use. I like the idea of the size ranges mentioned above but certain, taller tanks, may need a more intense light (T-5 or MH) to properly grow "carpet" plants like glosso or HC. I would also like to see the width (front to back dimension) taken into consideration as well.


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## hoppycalif (Apr 7, 2005)

In the past I have argued that watts per gallon works no matter what the tank depth is, but:

For example - take two 100 gallon tanks, one with length and width of 20 inches by 20 inches, therefore with a depth of 100 x 231 divided by 20 x 20 equals 57.75 inches.

The second tank with length and width of 30 inches by 30 inches, therefore with depth of 100 x 231 divided by 30 x 30 equals 25.7 inches.

With both tanks having 200 watts of light. Both have 2 watts per gallon. Assume the light over each tank is a MH light that is significantly shorter than the tank top dimensions. The light intensity from that light will drop proportional to the distance from the bulb squared. So the deeper tank will have an intensity at the bottom of the tank that is 25.7 squared divided by 57.75 squared times the intensity at the shallower tank. That is about 1/5 th of the intensity.

The problem is that tank volume is proportional to tank depth, but light intensity at the bottom of the tank is proportional to one over the tank depth *squared*.

You can't really come up with a definite formula to account for tank depth in the watts per gallon requirement because the drop in light intensity is only roughly proportional to one over tank depth squared and only for light bulbs that are small in both dimensions compared to the tank top area. For a thin, but long bulb, like a T5 bulb, the drop is much closer to directly proportional to one over the tank depth. PC bulbs would be somewhere in between.

I think what is important is that the "rule" of 2 watts per gallon is referring to tanks of "normal" depth, and with lights shaped like a T8 bulb the same length as the tank length, and, also for PC bulbs with reflectors comparable to AH Supply reflectors. Then, if you use T5 bulbs you can divide the 2 watts per gallon by something like 1.3. And, if the tank is twice the "normal" depth, you need at least twice the 2 watts per gallon.

Even with those "corrections" you are still just crudely guessing how much light is needed - if your aquascape doesn't feature a carpet of plants and does feature a high driftwood centerpiece, the adjusted 2 watts per gallon will cause serious algae problems on the upper parts of the driftwood. And, if you want a really low carpet of HC, for example, but don't have any high stem plants or high rocks or driftwood in the tank, the adjusted 2 watts per gallon could be too low.

Perhaps some enterprising vendor will "develop" a divining rod that we can carry to the LFS and have it pick just the right light for us! I won't suggest which vendor will be first to offer it.


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## Tex Gal (Nov 1, 2007)

hoppycalif said:


> Perhaps some enterprising vendor will "develop" a divining rod that we can carry to the LFS and have it pick just the right light for us! I won't suggest which vendor will be first to offer it.


Wouldn't that be nice!!!

It's so complicated. I think we may be doomed to the "What light should I have ?? forever." Even with Hoppy explaining so well his ideas, he lost me. :noidea: I bet a lot of you knew what his figures represented. That puts a lot of pressure on you all that do get it! [smilie=n:


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## MatPat (Mar 22, 2004)

Excellent info Hoppy, even if it is a little hard for us "common folks" to grasp  Tank depth has always been a factor in lighting a tank but most of us did not really know how or why and I think you have explained that very well for us. For those of us who are mathematically challenged, how much of a difference in light would there be on a 75g tank if someone were using a fixture (insert your preferred fixture here) placed directly on top of the tank or using the same fixture hanging at, say, 4" or 6" above the tank? Would it be a dramatic change in the light level or would it have very little impact?

As Tex gal stated, lighting can get very confusing and I would love to see some more info on the *approximate* levels where CO2 becomes a necessity. Light is the main driver in both plant and algae growth and will pretty much determine all other factors related to plant growth in our tanks. Like I mentioned earlier, I see a lot of threads where 2wpg is suggested as a starting point. Most of this advice doesn't take into account the type of lighting the individual chooses to use over their tank. I think this can be disastrous for the beginner who is told that 2wpg of T-5 light will not require CO2.

I would agree with the old wpg "rule" that if someone is using 2wpg of T-12 lighting in a standard shop light, this is about the point where they would need to consider adding CO2. So, for the various other type of lighting out there when does the addition of CO2 become a necessity?

In my past experience with T-8 shop lights, CO2 becomes a necessity somewhere in the range of 1.5-2wpg.

How about the AH Supply Compact Fluorescent fixtures...these are reported to have excellent reflectors and can get a lot of light into the tank...so what is a good recommendation for this style of lighting? Is it somewhere in the range of 1-1.5wpg?

Does CO2 become necessary when using 1wpg of T-5 light with individual reflectors or does CO2 need to be added with even less than 1wpg?


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## BryceM (Nov 6, 2005)

In addition to tank depth, there are some other factors that come into play with regards to tank size. People have stated for a long time that large tanks need relatively less light and that small tanks need relatively more light than aquariums of standard size (20g-60g). This link is an interesting analysis of the lighting that Amano has used in various size aquariums. It is an imperfect analysis, but it demonstrates to me that large tanks can get by on far less light than smaller ones, even if there is a greater average distance from light source to substrate in bigger aquariums.

How could that make sense?

It probably has something to do with the ratio of volume to footprint. Watts per square foot of surface area is probably a better measure of lighting requirements than watts per gallon.

I've posted this here before, but I think it's interesting to consider:

A standard 10g aquarium has a footprint of 20" x 10" or 1.39 square feet. 30 watts of light over this tank (3 watts per gallon) would provide 21.6 watts of light per square foot.

A 180g aquarium has a footprint of 72" x 24" or 12.0 square feet. 540 watts of light over a 180g tank (again, 3 wpg) would provide 45 watts per square foot, more than twice as much light.

Go to the extreme:

A 2.5 gallon tank is 12" x 6" or 0.5 sq feet. 7.5 watts / 0.5 sq feet = 15 watts per sq. foot.

A 300g tank is 96" x 24", or 16 sq. feet. 900/16 = 56.3 watts per sq. foot. The difference now is a factor of 3.75.

I think this partly mitigates for the dropoff in intensity that happens as the substrate gets further from the light source. The 2.5g aquarium is only 8 inches deep while the 300g one is 30 inches deep.

So, after all that, it remains a complex issue. For the most part, lighting is a huge variable that is determined only once at the time of initial setup. To establish a well-balanced system, everything else in the tank must be tuned to match that parameter.


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## hoppycalif (Apr 7, 2005)

Since the light intensity drops approximately by the "inverse square rule", meaning that every doubling of the distance from the bulb drops the intensity by a factor of 4, it has to be true that a pendant fixture hanging high above the tank provides a lot less light than that same pendant hanging right above the water.

But, the higher hanging pendant provides much more uniform light in the aquarium. For example, say you have a pendant hanging 2 feet above a 2 foot deep tank. At the surface the light is 2 feet away, and at the substrate it is 4 feet away, so the ratio between the intensity at the substrate and at the surface will be 1 to 4 - 25% of the surface intensity at the substrate. But if that light is 1 inch above the water surface (forgetting that this only applies directly under the light) the drop in intensity will be 1 divided by 25 squared or you would have only 0.2% of the surface intensity at the substrate.

This should explain why:
People get by with 250 watt MH pendants over 50 gallon tanks.
People get very good results with that light.

This works because the light is suspended high enough to drop the intensity at the water level low enough not to fry the plants. And, being suspended that high means the intensity doesn't drop off much at the substrate level.

This almost convinces me to buy a 250 watt pendant!


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