Carbonate Hardness levels (KH)

> From: Calvin Chin <cfc at pacific_net.sg>
> Date: Sat, 19 Oct 1996 00:06:29 +0800 (SGT)
> Dupla recommends a KH level of 3.5 at least, to prevent PH fulctuations they
> say.

I might be wrong, but I think they recommend 3.5 dKH so that their
Continuous Test reads somewhat correctly.  In general, I've had
strange results with that little piece of equipment.  A real CO2 test
might serve you better.

> Since I am using a Dupla continuous PH control unit tied in with my C02, I
> was wondering if a KH level of 2 will be sufficient. With a KH of 2, I do
> not encounter wide PH fluctuations at all and C02 level is just sufficient
> (Dupla C02 monitor gauge reads green - sufficient C02). 

You might approach this from a different direction.  

1. Dissolved CO2 levels should be 15-20 ppm for optimum plant growth
   and minimal distress to the fish (IMHO, of course). 

2. Pick your desired pH.  

3. Consult a pH/KH/CO2 table to see how much KH gives you the
   preceding values.

4. Let your controller work on that.

For example, with 15 ppm of CO2: 

    pH 6.6 requires 2.0 dKH
       6.7          2.5 dKH 
       6.8          3.1 dKH
       6.9          4.0 dKH
       7.0          5.0 dKH