Yeah citric acid will feed and promote bacterial life. Most likely where the foam is coming from. Why organic dudes usually use it. Prob not a good choice for RDWC or hydro in general.
Phosphoric acid is what you want to use. Also provides K to the plants. As much as it pains me to say this I suggest
Advanced Nutrients PH Down. What I have always used. It's the most concentrated you will get off the shelf without having to fuck with really caustic acid (unless you are comfortable with that). 1 cap full will drop 40 gals down from almost 10 after silica to around 5.8ish for me. The other good one is the GH Dry PH down. The liquid PH down from them is trash and crazy watered down. I would go through a gallon in 2 weeks. I use their dry up when I overshoot. Gallon of Advanced will lasts me about 5 months plus mixing 160 gallons plus a week.