This may be true, especially if you have no surface agitation. You can add that with an airstone. I have a heavily planted tank, and I keep an airstone running 24/7. I find that it stabilizes the ph swings between night and day by providing a continuous CO2 sink to the atmosphere. Of course, I don't use a controller. I researched the subject and almost bought one, but decided it didn't add enough value to justify the cost, and having the controller lowered reliability of the completed system too much. I'm not arguing with your decision to use one, I just couldn't justify it for my own use.