hulkamaniac said:
Anyway, the tank is 75g (I always forget to add that). I'm even more confused after asking, but that may be just me. I assumed you could find out the co2 in your tank in PPM based on the Ph and the KH in your tank. Since CO2 bonds with water to form carbonic acid, then it would seem to make sense to me that you could measure the ammount of CO2 in the water by the change in PH if you knew the KH before hand. I've got one of those KH testing solutions that uses the vial of water and a dropper. Is that a bad method of testing?
Also, I was under the impression that the levels of CO2 don't affect the levesl of O2 in the tank. Am I incorrect on that point?
In a perfect tank with few dissolved solids the KH/pH relationship would work. But that is rarely the case, especially when you start dosing ferts. All of these additives have an affect on the pH and so while your KH might be correct, the measured pH (and thus extrapolated CO2 level) will not be correct. The degree of error is specific for that individual tank and can vary quite a bit.
The drop checker method eliminates all the guess work and variation. You know the liquid in the drop checker is of a certain KH and contains no other additives that could throw off the reading. It's isolated from the rest of the tank water, but is directly affected by the dissolved CO2 in the water.
If you already own one, I highly recommend using it along with the pH/KH method.
The levels of CO2 and O2 are completely independent of each other. You can have high levels of both, low levels of both, or a mixture of the 2. In a CO2 injected planted tank the dissolved CO2 and O2 levels during lighted periods are normally both high. The CO2 is artificially high because your injecting it, and the O2 level is "naturally" high because the plants are giving it off. This is readily apparent when the plants pearl (O2 saturation causing the bubbles to appear in the water).
The problem then comes when the lights go off. The same plants that were previously consuming CO2 and releasing O2 are now essentially doing the opposite (simplifying here). That means they are consuming O2 and giving off CO2. In a low flow situation or where the hood is tightly closed you can have an oxygen deficiency in the tank, AND a CO2 excess. This is not a good situation. This is why many people shut off their CO2 after the lights go out, and why many also have air pumps turn on after lights out to keep the O2 levels elevated in the tank.
A quick lesson on CO2 and O2 in respiration:
Low O2 levels are bad (obviously) because per breath your lungs/blood are getting less oxygen. Lowering the respiration rate by doing less will help alleviate this. Think about high altitude climbers. They can rest for short periods of time during a climb to re-oxygenate the blood/tissues. Your fish can do a similar thing, and fortunately most fish that are not nocturnal going into a lower "power" state which requires less oxygen. Too low is self-explanatory.
Now CO2 is a completely different beast. We don't care (fish wise) about low CO2 levels. If the water is deficient in CO2 and high in O2 your fish are loving life. Same with low CO2 and normal O2. If the water is high in CO2, however, we have a very bad situation.
When we or fish respire we use oxygen and give off carbon dioxide. The blood in the aveoli (gas sacs in the lungs) comes into contact with the air/water and CO2 diffuses into the air/water, and O2 diffuses into the blood. This is a PASSIVE DIFFUSION. That means there has to be a gradient, or high level on one side and low level on the other. We never have a problem with O2 levels since there will (normally) always be less O2 in the blood then in the atmosphere. This then readily moves from the air/water to the blood.
CO2 operates in a similar fashion. When CO2 levels are higher in the blood then the atmosphere (this is usually the case) the CO2 leaves the blood and goes into the air/water. Everyone's happy.
Now we move to the heavily planted tank when the lights turn off. The plants which had previously been saturating the water with O2 now start consuming the O2 and releasing CO2. So are the fish. So you have a net loss of O2 and a net gain of CO2 in the tank. Remember that respiration is passive, it only works properly when the level of the "bad" thing is higher inside the blood and lower in the air/water. You've got the plants releasing a lot of CO2 and the fish doing the same thing.
What eventually happens is the level of CO2 in the water begins to get dangerously close or surpass the level of CO2 in the blood. When this happens there is no CO2 transfer out of the blood and bad things start to happen very quickly. And remember, unlike low O2 levels, you can't relax or breath faster to get the O2, all that does is speed up the process of asphyxiation (ie death). I like to think of it like carbon monoxide poisoning. There are minor differences between the two, but both end with the same result.
So keep those O2 levels up, but more importantly make sure you don't get high CO2 levels when the lights aren't on. You fish will thank you.
HTH
I'm sorry but that is quite funny! 
