It is a question often asked by many hobbyists new to CO2. Should I use a pH controller? Many experienced plant keepers will tell you it is not necessary, I am one who agrees. However, CO2 is one of the most difficult parts of aquascaping to master. It is also one of the most crucial for ensuring healthy lush growth. A pH controller when used appropriately and when considering the various limitations of the method (as you should with all methods) it can be helpful in CO2 addition. I hope to outline a method I use to limit the shortfalls of using a pH controller and get healthy plant growth. What is a pH controller? A pH controller uses a pH probe to measure the water pH and control the function of some other piece of equipment. In the case of growing plants it is used to turn on and off CO2 injection at a preset pH measurement. For our purposes, pH is simply a measurement on a logarithmic scale ranging from 0-14. 0 being the most acidic and 14 being the most alkaline. A quick breakdown of pH can be read here. As CO2 is added to the water it is dissolved by binding with water molecules creating carbonic acid. The increase in acid lowers the pH value. KH and buffering capacity KH is a measure of carbonate hardness (CaCO3). Calcium carbonate acts as a buffer when CO2 is added to the water. It essentially neutralizes the additional acid preventing a change in pH. This does not, however, alter the amount of CO2 in the water, it simply prevents a large measurable change via a pH value. A buffer will not completely prevent the pH from moving. It will, however, reduce the range to which the pH will change when a given amount of acid (CO2) is added. For example, water with a low KH will show a larger pH drop when a certain amount of CO2 is added than water with a higher KH with the same amount of CO2 added. All buffers have a limited capacity meaning they can only absorb so much acid until the buffer is exhausted. At this point small amounts of acid can cause large changes in pH. This is why it is recommended to always have some level of KH in an aquarium. 4 dKH is generally the lowest recommended level to maintain good water stability and avoid a pH crash. It is essential to have a very good idea of the buffering capacity through a reliable high quality KH test in order to estimate your desired pH value and likewise desired CO2 concentration. Common hobbyist test kits are unreliable. I recommend a Lamotte KH titration test kit. It is very easy to use and has an indicator color that is easy to read and changes if you have added too much reagent letting you know to retest. A video tutorial on using a Lamotte test kit can be found here. The relationship between pH,KH, and CO2 ppm How does knowing pH and KH allow us to estimate our CO2 ppm? Let's look back, if we know adding CO2 changes our pH and that KH will absorb some of that change in pH we have two of three variables. When knowing two of three variables it is possible to solve for the third. This is where the math gets crazy, grab a pen, a calculator, and an abacus. You will need them. Just kidding...thankfully there are many charts available to use for reference. Here is a nice article from Tropica with a chart. The limitations of using pH, KH, and CO2 chart When referencing the chart it is important to keep in mind that there are a few limitations in using this method. The first being it assumes there are no other acids in the aquarium environment altering the pH. However, as many of you may know this is not the case. As with all biological systems, artificial or natural, there will be some sort of organic acids from biological processes and the decay of organic matter. Driftwood can leach tannins into the water, some aquarium planting soils also reduce pH. In short there are other things reducing the pH. The other assumption is that your KH remains stable. If you are using seiryu stone or another type of stone that has a tendency to alter water chemistry this will not be the case. Your buffering capacity can change over time. Both of these things can skew your estimation. Interestingly they counteract each other, but it is too difficult to estimate to what extent and would vary between systems. In order to reduce the margin of error from these limitations I measured my KH immediately before a water change (after 10 days), which measured 14 dKH. This particular scape has a large amount of seiryu stone. The measurement immediately after a water change was 7 dKH. Now I have an idea of how large of a swing I would expect to see. I felt the range was too large and have since begun doing two water changes per week in order to limit this swing and maintain a more stable KH. To compensate for additional unknown acids, I set the controller to a point two boxes lower than my target. This put the estimation into the red zone. Interestingly previously I was overdosing CO2 as compared to the controller method as measured by my yellow drop checker. It is now green. A final limitation to consider is the reliability and reference of your measuring equipment. How do I know my pH controller is good? It is important to regularly calibrate your controller using reference solutions. I also use a drop checker for a quick visual reference and second measurement. This way I may see something is wrong before it starts to manifest as algae or poor plant growth. Lastly, I used a handheld pH meter to corroborate my probes measurement. The handheld meter was independently calibrated and used one day after probe calibration. The two meters matched, increasing my confidence in the probe measurement. A word on limitations of drop checkers: Drop checkers have a lag time in their reading meaning the color you see now may not be representative of the actual CO2 concentration at that moment. Drop checkers rely on the same pH, KH, CO2 relationship but do it within the drop checker. CO2 must degass into the drop checker and redissolve into the solution within the bulb. The subsequent change in pH causes the indicator solution to change color. This all takes time. It is further compromised by the method of CO2 distribution. Any methods that cause lots of CO2 bubbles like atomizers or diffusers will enter the chamber and cause an overestimation of CO2 concentration in the tank. For this reason, when using atomizers or diffusers and a drop checker I generally shoot for a more yellow green than if I were using a reactor. Also of importance is the location of your drop checker. It is best to locate it far from the source of CO2. I also try to place it in the position least likely to capture rising CO2 bubbles. These are areas of downcurents or behind an outlet. A nice article on drop checkers can be read here Advantages to using a pH controller Using a pH controller may provide an aquarist with greater control, increased efficiency, and demands a greater understanding of CO2 and its relation to other factors fostering improved CO2 management. Prior to using the controller option I would simply inject at an estimated rate and observe if it was sufficient for plant health. I would use my fish as an indicator of too much CO2. My drop checker was yellow, fish behavior was not as active as it is now with reduced CO2 injection. Plant health has not suffered. It can sometimes be difficult to find a rate that provides adequate CO2 at lights on and yet does not begin to overdose CO2 as the light cycle progresses. A CO2 controller can allow for a reduced injection rate started earlier and then maintain the desired ppm of CO2 throughout the light cycle. I should mention there is no reason to turn CO2 on when the light is off with or without a controller. Information Page for pH and pH probes from Milwaukee Instruments pH info and probe testing See the probe test. A good way to determine if your probe is functioning appropriately or needs replacing Information on usage and storing from The Barr Report A brief word... I use less CO2 with what seems to be better results. One caveat to this improvement. Prior to using the pH controller feature I had purchased a PAR meter in order to dial in my lighting to the exact level I wanted it. Light is easily controlled, but hard to measure without a PAR meter. Once the light level is determined to be sufficient and not overkill, CO2 was easier to get right. Light is still the paramount factor in determining CO2 demand. Fortunately it is also the easiest to control. *I will continue to edit and update as time allows. I hope this is helpful to those wanting to try using a pH controller. All photos are my own. Finally, remember a successful planted aquarium/aquascape depends upon many factors: filtration/flow, CO2, light, fertilizing, and maintenance. Without all components working well together success is unlikely. There is usually more than one way to skin a cat, this is how I skinned the CO2 cat on my current 90 cm scape. Past scapes did not use this method.