Hardness question

George said:
> > I have a question: when is it appropriate to use calcium carbonate
> > instead of or in addition to sodium bicarbonate? How much should be
> > used and is it used up? 
> Sodium bicarb will increse the KH (acid buffering, alkalinity) and add
> some sodium ions to the water. Calcium carbonate will raise both KH
> (via the CO3) AND the GH (general, calcium hardness).  Which one or
> what kind of mix depends on your tap water and desired aquarium KH and
> GH.  

Guess I'm asking the wrong question here. When do I want calcium hardness
increased? Virtually all plant descriptions indicate they do fine
with a very low hardness. Does hardness cause plants or algae to
slowdown in their processes? Maybe more calcium hardness would decrease
my problems with brush algae. I simply don't get the same results
as everyone else with plants competing with the brush algae. With
higher light levels (250w MH on the 49g) I have lots of thread algae
and with a recent CO2 deficiency, the pH is upto about 7.2 and the
brush algae is growing fast on some plants.

> One teaspoon (about 6 grams) of sodium bicarbonate (NaHCO3) per 50
> liters of water will increase KH by 4 degrees and will not increase
> general hardness.  Two teaspoons (about 4 grams) of calcium carbonate
> (CaCO3) per 50 liters of water will increase both KH and GH by 4
> degrees.  Different proportions of each can be used to get the correct
> KH/GH balance dictated by the fish and plants in the tank.  Since it
> is difficult to accurately measure small quantities of dry chemicals
> at home, a test kit should be used to verify the actual KH and GH that
> is achieved.

The only hardness kit available seems to be general hardness. I bought
one a few years ago (Tetra) and it never worked properly. What's a good
brand and do I need it? Can't I just stay with low Ca/Mg? I suppose I
can calculate carbonate hardness from pH and CO2 but I need an
accurate pH kit for that. Bromothymol blue just isn't accurate enough
IMHO. Anything better? Does the Lamotte kit use titration? Is there
a good carbonate test kit? If we control CO2 & pH closely (say by
spending the $ for a pH controller), do we need to increase carbonate
hardness either?

Something Paul said about calcium carbonate requiring CO2 in solution
in order to dissolve makes me think it might make a better pH buffer
than sodium bicarbonate which dissolves easily and would cause an
immediate increase in pH. Calcium carbonate would not dissolve as
much, hence the pH increase should be less (?). As CO2 gas was added
to the system, the pH changes should be less overall. Is this right
thinking or all wet? ;-)