Many things can be happening. Citric acid is a good nutrient for microbial life. If your system isn't designed or treated to remove all microbial life, you may well be culturing bacteria. You may want to culture bacteria, to aid plant roots in the absorption of inorganic nutrients, or you may have a system using filters and treatments to remove algal contamination, but that my not help citric acid persist.
Using phosphoric or nitric acids is a good way to adjust the pH -- if you want the pH to fluctuate. On some level you do -- if you put the acid injector close to the pH sensor, you will get frequent acid injections, that stop further injections, that the plants will then absorb rapidly, again causing wide swings on pH, but increasing the health of the plants.
You shouldn't be visualizing your hydroponic system as a vat in which chemical reactions take place, where we try to make conditions as static as possible. It isn't even like a field of grain, in which a farmer essentially "sets up" a field as if it were a large very open vat -- with tilled soil, appropriate pesticides and fertilizers, the best irrigation system they can afford (and are legally allowed based on water use laws,) and then hopes the weather holds out. Under high light and temperature conditions, it is reasonable to expect daily or even more frequent fluctuation or components of your nutrient solution. And you should definitely expect a biological explanation over a chemical explanation, unless stuff is precipitating in your holding tank or conduit lines.
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Topic: Citric acid ph drift (Read 3896 times)