Alkalinity, pH, CO2 triangle

Hi everyone,

Hope nobody's hangover was too severe...

After mine went away, I spent some time this afternoon testing the water in my newly set-up 120 G tank, using the LaMotte test kits Santa dropped off.

The tank was set up on Dec. 22 and I used 50% R/O water and 50% tap water. Generally, I am quite happy with the tank's progress so far. The initial cloudiness of the water (caused no doubt by the fact that I placed some Red Art clay in the lowest layer of the substrate) cleared up in only a couple of days and the water, while not yet crystal clear, is clear enough to see easily through the full six foot length of the tank. Today I noticed the very first blush of green algae on the front glass of the tank. It comes off easily with a soft pad and there is no trace of algae on the plants in the tank. The plants have started to grow, especially the hygro, but I haven't noticed any O2 bubbles forming on the leaves yet so I guess that they are still in the process of establishing root systems.

The test results that I measured today were as follows:

Temp.		80 F
pH		7.4			(I have the Narrow Range pH kit which reads from 6.0 - 7.4) 
Alkalinity	64 ppm CaCO3  
CO2		7 ppm		
Ammonia	0 ppm Ammonia-Nitrogen
Nitrate		2 ppm Nitrite-Nitrogen
Iron		0.6 ppm Iron
Phosphate	0.2 ppm Orthophosphate
Oxygen		76 ppm O2

As the tank is illuminated by 3 - 175W 5500K Metal Halides, on for 12 hrs. per day, I am assuming that the fact that the Oxygen concentration is at slightly less than saturation due to the fact that the plants are not yet fully established.

But I am wondering about the CO2-pH-Alkalinity triangle. I have not yet set up the equipment for CO2 addition to the tank, due mainly to the holidays. I have installed the Dupla Visual CO2 test and it has remained Green (indicating that the CO2 level is O.K.) since I put it in the tank several days ago.

The Dupla CO2 Table indicates that, given a pH of 7.4 and an Alkalinity level of 64 ppm CaCO3 (3.76 degrees KH) that my CO2 level should naturally be at approximately 4.5 ppm (and therefore deficient). I once again assume that once I begin to add CO2 to the tank from my new compressed gas cylinder, that the pH should begin to fall from it's present level (I am going to aim for a pH of 7.0).

How accurate is the Dupla CO2 Chart? Given my pH of 7.4 ppm and the Alkalinity level of 3.76 KH, I would expect to see only 4.5 ppm CO2

Several sources, including Dupla, indicate that the presence of humic acids in the tank water can throw off the results of CO2 tests, showing falsely inflated readings. I placed approximately 1 kg. of Sera Peat Granules in the lowest layer of the substrate (mixed with the Red Art clay). This is covered by 300 lbs. of gravel, giving me a substrate depth of between 4 and 6 inches thoughout the tank. I realize that the humic acids released by the peat could have percolated up through the gravel, and might be affecting the Dupla CO2 visual test, but would it have the same effect on the LaMotte CO2 Test as well? And if the higher CO2 levels are the result of leakage of humic acids from the peat, why hasn't the peat also brought down the pH of the water? (My tap water has a pH of 7.4 and mixing R/O water 50-50 shouldn't change the pH)

So how detrimental ARE humic acids to this equilibrium? Are my readings bogus (I was VERY carefull when I ran the tests and the LaMotte kits are supposed to be accurate) or could something else be causing the readings I am getting (like am I pushing the pH kit beyond it's limit - I know that they aren't as accurate at their limits)? 

Or am I being anal, and should I just relax and let the tank settle into it's own rythym?

Chemists, start you engines please.


James Purchase
Toronto, Ontario
jpp at inforamp_net