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Re: buffering soft water for CO2 system

> Date: Sat, 4 Jul 1998 17:06:42 EDT
> From: RDotta7777 at aol_com
> When I hook up my CO2 system (using a pH meter to trigger the CO2 injection),
> the water after a water change has a pH of approx 6.2.  I then allow the CO2
> to bubble in (at a slow rate).  In about 15 minutes my 125 gallon tank has a
> new pH of 6.1 and the system shuts off.  The pH never comes back up to re-
> trigger the CO2.  I have to add Sodium Bicarbonate to re-adjust the pH to kick
> the CO2 back on.  I have to do this every day.  Is there something I am
> missing or is this just the way one has to add buffering into the system in
> soft water?

In your situation, this is "the way". Let me explain ...
> I only keep South American dwarf cichlids and other South American species, so
> the soft water is great.  But I like a well planted tank and would like to get
> the most out of the CO2 system.

If you are going to try to control the pH/hardness of your tank, it would be 
wise to first get the Tetra GH and KH hardness test kits. They are inexpensive, 
accurate enough for aquaristic purposes and easy to use. 

The "soft water" you desire dictates low concentrations of calcium and 
magnesium. Tetra calls this "General Hardness" or GH. GH is what affects the 
health and well-being of the internal organs your fish. Proper calcium levels 
allow the cells in your fish to develop properly. 

Sodium bicarbonate does NOT affect GH. 

If you want to "buffer" your water to control pH, you need to be concerned about 
the alkalinity. A major component of alkalinity in natural water is carbonate or 
bicarbonate ions depending on pH. Lower pH water will have predominately 
bicarboante ions.  Tetra calls this Carbonate Hardness or KH.  Obviously, sodium 
bicarbonate DOES affect KH but does NOT affect GH. Calcium carbonate affects 

Most "KH" test kits (like the Tetra kit) actually meassure alkalinity.  But if 
you know your water is low in other alkalinity producing ions like phosphates, 
measuring alkalinity is a good way to approximate carbonate hardness. 

To properly use CO2, find one of the "CO2/KH/pH" tables that are on various web 
sites.  To use the table, first decide what pH you want and how much CO2 you 
desire. A good CO2 level for plants is about 15 mg/L. On the table, see how much 
KH corresponds to that CO2 and pH.  Then, add enough sodium bicarbonate to get 
to that kH level (which will raise the pH) and inject enough CO2 to bring the pH 
down to what you desire. Voila!

Since you are changing the pH with the sodium bicarb, add it slowly and check 
the results with the KH test kit as you go.  It is best to increase KH and CO2 a 
bit at a time, so you don't produce a large temporary change in pH. 

Unfortunately, you may find that the low pH you desire is not practical. For 
instance, let's say you want a pH level of 6.2 for your cichlids.  This pH with 
a CO2 level of 15 mg/L dictates a KH of 0.75 degrees (about 13 ppm for the 
"degree challenged"). This is a very low amount of buffering and would probably 
not adequately "fix" the pH. This is most likely the situation you already have! 

The observant reader might think, "But wait!  The chart says that for pH 6.2 and 
0.75 dKH, I have plenty of CO2!"  Close, but no cigar. The chart assumes that 
carbonic acid from the dissolved CO2 is the only thing balancing the bicarbonate 
ions. But at this low level of KH, other acids (such as those produced by 
nitrification) are also having an effect. The charts are most effective at 
higher KH levels (>2 dKH). 

Unfortunately, this leads to the conclusion that a "lower pH" plant tank with 
CO2 injection is going to be tough to pull off. So, you either have to go with a 
higher pH so that your CO2 system will be effective or go with a lower pH and 
continue with what you currently do. 

IMHO, your fish will adapt just fine to a higher pH as long as the GH is still 
low (i.e., adjust the KH with sodium bicarbonate). Others with more fish 
experience may want to jump in here. 
For further info on CO2 and a CO2/pH/KH table, check out my website. 

George Booth in Ft. Collins, Colorado
 Do you want to know how I did it?