Although CO2 is definitely needed for freshwater aquarium plants, too much CO2 will drop an aquarium pH which is very dangerous for fishes and other inhabitants in your planted aquarium. CO2 level up to 40 mg/liter (ppm) is considersed safe for the organisms.
As we know, fishes and crustaceans are very sensitive to pH fluctuations. In aquascape, fish tank plants take important parts in pH fluctuations - they absorb CO2 and expel Oxygen during the day (photosynthesis process), on the contrary, they absorb oxygen and expel CO2 during the night (respiration process).
As results, pH level will reach its lowest level in the morning (before the photosynthesis begins) and pH will reach the highest level at night (before the respiration process begins).
Some fishes like pH<7 (such as tetras, discus, angelfish), the others (such as live-bearer fishes, african chichilds) prefer pH>7. If you keep various kinds of fishes in a planted aquarium, you'll need to stabilize the pH level at 7 - the safest pH for almost all kinds of fishes.
In a new set-up aquarium where the photosynthesis process is not active yet, only low CO2 is needed. The need for CO2 will increase when the freshwater aquarium plants starting to grow, on the contrary, it will decrease after pruning your fish tank plants. So, the need for CO2 in a planted aquarium is changing from time to time. If you constantly supply the same amount of CO2 at all times, the planted aquarium will suffer from under/over supply of CO2 and it will greatly affect the pH!
A good aquarium CO2 system will automatically adjust CO2 being released, it has the magnetic valve that can open/ close CO2 flow depending on needs. The censor system will automatically combinates CO2 with the KH existence to get the desired pH (pH deviations are less than +/- 0.1). Although it's more expensive than others, what this system can do is worth the price. So, I recommend this aquarium CO2 system for your planted aquariums.
After some readings, you maybe concluded that pH is the most important factor here. Yes, it's true - pH is the King!
But CO2 is not the only determination factor here, as previously mentioned, KH (Carbonate Hardness) also takes the important role here - there is a well defined relationship between KH, CO2 and pH.
KH (carbonate hardness) is a pH buffering, it prevents pH drops by its acid absorbing capacity, as long as KH is in a good level. Please read the article which specifically discuss the issue. Actually, it's just a simple rule : adding KH increases pH, adding CO2 lowers pH, a good KH level provides a stable pH and the CO2 injection allows us to set a specific pH.
For example (see the above chart):
If your water has 1.0°dKH and 3 mg/l of dissolved CO2, your pH will be 7.0.
1.0°dKH is not a good KH level, you should add sodium bicarbonate/ KH Buffer to get to the moderate KH level, like 4.0°dKH for a good buffering. The pH will increase to 7.6, but if we inject enough CO2 to give us 11.8 mg/l (ppm), the CO2 will bring the pH back down to 7.0.
The CO2 and KH balance each other and we have:
a neutral pH (7.0) which is suitable for almost all kinds of fishes and freshwater aquarium plants.
a good buffering to resist pH changes, giving a stable environment.
a good dissolved CO2 for great freshwater aquarium plants' growth.
If you need to add dissolved CO2 (ex: to 14.7 mg/l) due to the nature of the fast-growing type of fish tank plants, you should also adjust the KH level by using KH Buffer (to 5.0°dKH) to get the same pH.
Important notes:
Do not try to change CO2 or KH level by changing pH (such as adding acid or other buffers besides carbonates)!
Check if there are any other buffers besides calcium carbonates (CaCO3) in your aquarium water. The presence of other buffers (such as phosphate) should be eliminated first, before you try any adjusting efforts.
After you understand the water chemistry, in fact, aquascape is not that complicated. Practice what you learn & learn from your mistakes. Practice makes perfect.
Aquascape's Digest 
to get to the moderate KH level, like 4.0°dKH for a good buffering. The pH will increase to 7.6, but if we inject enough CO2 to give us 11.8 mg/l (ppm), the CO2 will bring the pH back down to 7.0.
