Hi,
In my opinion, while PH controllers may seem like a good investment, in reality they can sometimes cause more trouble than they are worth.
The first problem is that the probe can only measure the pH of the tank water. It is a fundamentally flawed idea to correlate tank water pH with CO2 concentration. The reason for this is due to the fact that there are many acids in the tank water other than Carbonic acid formed by CO2. For example if you dose Potassium Phosphate (which you should be doing) the phosphates form phosphoric acid. There can also be tannic acids released by wood, and a host of other types of organic acids. If you measure the pH of your tank water right now and use that value to calculate the CO2 concentration it is a guarantee that the value will be artificially high.
The second problem is in the way that the pH is regulated. In the beginning there will be a large injection to bring the pH to within the set point, then the controller will shut down or slow the injection rate. As the pH rises and falls above or below the set point the controller will respond to the fluctuations by raising or lowering the injection rate. The result of this action is that the CO2 concentration is rarely ever stable. Fluctuating CO2 concentration brings the onset of some types of algae.
Plants produce a type of enzyme which responds to the CO2 concentration level in the environment. This enzyme, acting as a sensor allows the plant to regulate all of it's other functions (such as photosynthesis, growth, food production and nutrient uptake) based on the concentration level. If the level is stable then these other functions are also stable, but unstable CO2 levels has a devastating effect on the plant metabolism. A pH controller therefore contributes to unstable CO2 levels by the mere fact that it continually varies the injection rate. It is much better to have a stable injection rate set to a high enough level to satisfy the plant demands throughout the day while minimizing the fluctuations. The pH of an aquarium has been given too much priority in relation to the other tank parameters. Plants simply don't care about the tanks pH as much as they do about nutrient availability and CO2 concentration levels.
Cheers,