Why CO2 once over 3 watts/gallon?

Puffernewbee

Puffernewbee

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Dec 9, 2004
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People always say that once you get over 3 watts per gallon you need CO2 for yur tank. Why is this?
I want to convert my tank to planted but when looking for lighting fixtures it seems that with my 72 gallon seems to be the awkaward size. Lights seem to start at a low with 2X65 watt bulbs (1.66 w/g) and then jump to 4x65 (3.333 w/g). A&H Supply has 4X55 but that still gets me over 3 w/g.
 
Larger tanks do provide and extra complication as well in that they are generally much deeper. So if they are NO flourescent lights their depth penetration isn''t perfect, so plants at the bottom get less light. So 3 wpg probably isn't quite as much as in a shorter tank while 1.6 wpg is way too little for a heavily planted tank.

With too much light and too little CO2 you run the risk of the algae being able to outcompete the plants for resources.

For my 29 gallon (a tall tank) I decided to build my own lighting fixture as the options available weren't perfect. Perhaps you can do that as well.
 
In your standard "stock" tank plant growth is limited by two factors: light and carbon.

Once you remove the light limitation then growth becomes carbon limited. The plants want to grow faster, but cannot fix carbon rapidly enough. They are able to convert bicarbonate to useable carbon (a process called decarboxylation), to varying degrees depending on species, but the enzyme required to do so (Rubisco) is the slowest enzyme known, so decarboxylation is not the favoured pathway for carbon uptake. There's more to it than that, such as enzyme saturation to speed the process and a few other tricks that plants imply to get around the slowness of Rubisco, but the bottom line is that when ample light is supplied, growth becomes carbon limited, so CO2 is supplied to prevent that.

As for the wpg threshold, that's a little more hand-wavey, and very subjective. I would put the boundary around 2-2.5wpg. Watts per gallon is a "rule of thumb" based on the light output by a T12 NO fluorecent bulb and comparing it with the amount of light required to saturate photosynthesis. This, of course, has nothing to do with watts really, which is simply the amount of electrical power required to run the bulb, not the amount of light produced. Power compacts (PC) produce more light per watt, plus if you get the AH Miro reflector you're further inreaing the amount of light in the tank. So the type of bulb, or lighting system, becomes important.

Tank size is also a factor, as you amy guess. Since we're talking about the amount of light required to saturate photosynthesis our concern becomes an issue of amount of light per unit area (and there's a factor of scatter in there as well based on tank depth). However, after you get much bigger than a 75 gallon tank, surface area isn't increasing that much compared with the amount of light you are pumping into it, so the wpg rule of thumb breaks down in larger tanks. In tanks smaller than about 15 gallons the number of watts of light needs to be much higher to achieve photosynthetic saturation than would be required on a larger tank. So treat watts per gallon more as a very vague ballpark figure and work from there.
 
switches are necessary

I find that switches for the two banks of light are necessary to keep from having to run the full high wattage the whole day. In my tank, an odd sized 59 gallon custom tank, I have 4x55 watts, two switches. I run one line of lights for 11 hours, and add the second line for only 4 hours midday.
I have enough trouble keeping up with my ferts as it is, no need to push the plants to grow any faster, that's just more work!

Check with PetSupplyLiquidatiors.com for their light boxes, they have several that will work well for you. Run all over glass tops, even the Solar ones that have the plastic lens.
 
Thanks everyone for the info. Thinking of going towards a planted tank is information overload. :duh: It seems that you start to understand something about it when suddenly all that you thought you knew is only half correct. Either my brain will explode before I get started or my bank account once I get started.
 
Nash

You might want to come to a NASH meeting to learn about planted tanks from some of the best minds in Houston on the subject. Most of the mambers are serious about their planted tanks, some professional, but all will have sound advice and we trade plants for free. Next meeting is this Saturday at Aquarium Design Group, on W. Alabama inside the loop, I need to check on the time, PM me if interested.
 
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