the size of the tank doesn't affect the rate as there is a needle valve on the regulator that you turn to adjust the flow (say, 2 bubbles per second). You want to have the canister/regulator/bubble counter go to CO2 tubing (other kinds will leak CO2), to a check valve, then to either an inline reactor or a diffusor or in-tank reactor. The CO2 will lower the pH and also gradually eat away at the KH (buffering capacity). Most people however, have enough KH that it's not a problem, just check it every once in a while to make sure it's at a safe level. The CO2 should lower your pH by one full point (i.e. 7.2 to 6.2) if you have about 30 ppm of CO2 in the water (which is what you want). You will also notice a normal variation between first thing in the morning and late at night by a couple of tenths of a point. I don't use a controller (I don't like depending on pH probes that go out of calibration fairly quickly and there's an extra expense), so I leave my CO2 on 24/7. I make sure there is some slight surface turbulence so there is enough oxygen in the water at night when the plants are not photosynthesizing. Pretty much set it and forget it until the tank gets low 5 months later. Use the chart on Rex Grigg's website to measure your CO2 level using a pH and a KH test.