When I used diy c02, I always ran three 2 liter bottles and would change one out every week. This helped to maintain a more constant c02 level. If I didn't do this, I would get spikes when I changed out all the bottles and then it would level off low. Changing one every week stopped this. Also I would have all three bottles setting in a box or bucket with water and a heater to keep the water temp stable. Also I ran all co2 through another bottle as a bubble counter. This prevented co2 from entering the tank if things went wrong.
10 gallon hospital tank.
29 & 50 gallon - High Tech C02 enriched.
50 gallon Pond - Natural.