I don't know what size your tank is, but if it is 20 gallon or bigger, it isn't likely that you will overdo the CO2 with DIY system. If you think that could happen you can put an air stone in the tank and run it at night only - to add surface movement to outgas the CO2. CO2 drops the PH about 1 ph unit if you are adding about 30-40 ppm of CO2. You can check this by taking a sample of water from the tank, letting it sit at least 4 hours in a shallow glass, then comparing the PH in that water to that in the tank. The difference will be about 1 ph unit if you have 30-40 ppm CO2. If your KH is below 3 degrees, it is possible to add so much CO2 that the PH drops much too far - that is PH crash, and it is usually only a problem with pressurized CO2 systems. PH test kits are, in my opinion, pretty accurate no matter who makes it. The inaccuracies are largely due to the difficulty in comparing the color of the test tube of water to the color chart provided.