Excellent josh !!! I'd give ya two thumbs up but I can only add one here
The thing about CO2 - is the carbon.
There is a carbon cycle - just as there is a nitrogen cycle in our little boxes of saltwater sitting in our family rooms.
This carbon cycle is similar to the same carbon cycle that exists EVERYWHERE - including the ocean.
If there is one think to study in this hobby ( especially if running a reef tank ) after you understand the nitrogen cycle and how to cycle a new tank - it's this carbon cycle.
Carbon, hydrogen, oxygen, calcium, magnesium are the top 5 players in this crazy saltwater world we fiddle with.
It's the various COMBINATIONS of these elements - that both lead to a wonderful balance in the tankwater - AND - can reek havoc if they get out of balance.
Think about what these top 5 actually do ......
pH = amount of hydrogen in the tankwater.
CO2 = combined carbon & oxygen - used by algae for photosynthesis. Plants give off oxygen in return.
CO2 affects the formation of carbonic acid, which leads to formation of bicarbonates and carbonates - both of which help stabalize the tank's ability to neutralize naturally occuring acids as they form in the tank. CO2 affects the alkalinity level.
Carbonates are used by hard corals and coralline algae to grow.
Oxygen - used as fuel by most lifeforms - fish, inverts & bacteria.
Calcium - this one's obvious - it's allows for skeletal formations in coralline and hard stony corals. It also is a major factor in natural seawater - and must be maintained in our tankwater as well.
Magnesium - this one's trickier - but let's just say that without sufficient levels of magnesium - it's near impossible to maintain the alkalinity/calcium relationship. It competes with calcium - it's supposed to. It keeps things in balance.
And let's not forget about H2O - this one's importatant for sure ( I can hear you saying DUH~no kidding Broomer~ as I type this )
But it's the carbon cycle that really runs the balance.
Carbon in the form of CO2 that changes form - that creates CARBONic acid ....... biCARBONates ...... that form CARBONate.
The funny thing about CO2 gas and alkaline seawater/tankwater.
the carbon in the CO2 can actually become "entrapped" in the water - because it changes form into other chemical compounds.
But the reaction can go either way depending on a lot of factors.
The only way to measure total carbon in the tankwater ( or ocean ) is to measure ALL of the various forms that carbon can occur as.
CO2 being one - the gas we talk about.
Carbonic acid - tends to drop pH somewhat - but is natural process in the reaction and it's what allows a calcium reator to do what it does to the media.
Bicarbonates - aids in buffer capacity
Carbonates - aids in buffer capacity and donates itself along with calcium for coralline/hard corals to grow.
josh's answers are much more to the point Spruce.
As far as measuring CO2 and trying to control it - it's changing form so fast - that it's hard to measure.
But I will say this ......
The entire tankwater chemistry is affected by the AMOUNT of CO2 that either enters the tankwater from the air, is produced by living plants in the tank or is "free'd" up when the chemical reations moves from the right to the left.
Lastly - if you're interested - here's a very basic description of the reaction.
CO2+H2O <~~> H2CO3 <~~> H+ HCO3- <~~> 2H+ CO3--
CO2 = carbon dioxide
H2O = water
H2CO3 = carbonic acid
H+ = hydrogen
HCO3- = bicarbonate
2H+ = two hydrogens
CO3-- = carbonate
This chemical reaction can and does occur ALL THE TIME in our tankwater ~ and the actual amounts of each compound will exist in the tankwater DEPENDING on the relative concentrations of all the others.
I may not have it written exactly as a 'chemist' would write it ~ that's for sure
