I am currently running 4, 2-liter bottles of
DIY CO2. I change 2 every 2 weeks and seem to get reasonably good results. Right now I have the
CO2 coming into the tank in two places. One is a bubble ladder type setup, and the other is a diffusion bell. My
CO2 consistently test at about 22-26
ppm. I use the
CO2 chart to figure this based on Ph and Kh. Since I didn't believe my
DIY system could possibly work that well, I also bought a
CO2 test kit, and it gets the same reading as using the chart.
As for your Water, as Malkore mentioned, you have to know what you have before you start changing it. When you do test your tap water make sure you let it sit for a few hours after pulling it from the tap. There may be diffused
CO2 at first that will affect your readings, and you should let this outgas a bit.
Myself, I buy water from my local water store. It cost me $7.00 (US) a week to do 20 gallons
PWC's. their water is ultra purified, and tests at Ph 6.0, 1 degree of
GH, and 1 degree of
KH. From there I add my fertilizer mix. After my last
PWC (open's journal) my water tested at a Ph of 6.6 and 3 degrees
KH.
There are some other few things in my tank which I am sure affect the Ph/Kh to some degree, Such as African Bog wood which is very dark and rich in tannins. I also use a natural substrate from Jade Beach in Monterey,
CA. This has a modest sea shell content (Calcium) which I imagine has a small affect on raising the Hardness.