KH/pH/CO2 and Non-Carbonate Buffer Systems

From: Mike in St. Louis

> I'm wondering if somone can enlighten me on the concept of KH/pH/CO2 and
> the use of non-carbonate buffers. 

Non-carbonate buffers will give you false "KH" readings since most "KH
test kits are measuring total alkalinity.  If you use a table to
determine CO2 levels from pH and KH, the readings will be off.  If you
use a CO2 test kit, you should have reliable readings as long as there
are no strong mineral acids present. 

> I keep Discus in my plant tank and I like to keep my pH at around 6.6. 

Use sodium bicarb to get 2 dKH.  Inject CO2 to bring your pH down to
6.6.  You will have 15 ppm of CO2.  Since you have a pH controller, 
you don't need as much buffering.  With regular water changes
(refreshing any loss of KH due to nitrification), you won't have a
problem with pH dropping. 

Are you sure you really need to keep your discus at pH 6.6?  Many
successful discus keepers and breeders use whatever their tap water
gives them.  You may be causing more problems messing with the water
chemistry (and having it vary accidentally) than they would experience
if they acclimated to a higher pH. Ours are very happy at pH 6.9.