Skunky wrote:

OK, I've got my head around the relationship between KH, pH, and CO2.
Chuck's page was a great help!

Here is my concern. My tank runs at around 6.8 with a KH of barley 2
degrees. I would like the KH to be 4 degrees, I appreciate this will
put the R/O water stored in the vat to have a pH of around 8.0. I do
two water changes a week of around 80 litres (tank holds 260 litres) my
tank is well stocked, these water changes help to keep my NO3 below
30mg/l. Surely adding water with a pH of 8.0 to a tank which is 6.8 is
going to cause problems unless I start blowing CO2 into the R/O vat
aswell prior to adding it to the tank

KH2 is sufficient, albeit at the low end of the desirable range. Most plants grow in
it just fine for me. My water formula is:

72 gallon tank
20-25 gallons changed per week with reconstituted RO (no tap)
Per 10 gallons RO water: 1 teaspoon RO Right, 1/2 tsp baking soda

Result: GH and KH both 2-2.5
pH 6.4 with CO2 injection

Regarding the fact that the replacement water is higher alkalinity and thus pH than
the tank water, this is true in all virtually all injected tanks. Since my change
water is probablyat 3ppm CO2, which is the level at which water reaches equilibrium
with atmospheric CO2, my change water is probably about 7.3 according to Chuck's page.
This doesn't seem to have bothered the fish or plants in 18 months of weekly changes.

Remember that pH is a result of KH and CO2. You probably won't be able to precisely
predict the resulting pH of a water change. I have never measured the pH of my change
water. I just started with a bit less baking soda, changed the water, measured the
resulting KH and pH in the tank, and increased the baking soda in the next change
until my result was where I wanted it (KH 2-2.5).

My KH drops about 0.25 per week due to biological processes. If I started at KH2 and
missed two or more weeks of water changes, the resulting pH would start to drop below
desirable levels. But I've never gone more than 10 days between changes, so that's not
an issue for me. At a KH of 4, big trouble takes longer.