The first group of pH down chemicals are strong acids 😁 These are technically acids with very low pKa values, meaning they react instantly with water to generate at least one mole of hydronium for each mole of added acid 😎 They offer the strongest ability to drop pH per unit of volume, which makes them more cost effective 🙌 However the fact that they often need to be diluted to make the pH addition process practical – because of how much the concentrated forms can change pH – can make their use more difficult than other forms of pH down. These are the best options. 
Many growers also use potassium silicate as a part of their growing process. Potassium silicate, also known as K Sil, is extremely alkaline. This raises pH. It is necessary to adjust pH in order for K Sil’s increase to pH to work. A powder potassium silicate product (Agsil16H) was used to give 22 ppm Si (50ppm SiO2) in solution. After pH adjustment, Agsil16H added approximately 8ppm of elemental phosphoric acid to the solution. When considering that the P requirement of cannabis, during the height of flower (bulking/swelling), is about 75 ppm, 8 ppm of elemental P being added to solution through the use of phosphoric acid makes up about 10% of the plant’s P requirement. 
Update 2022: I’m having stopped using “organic acids” as pH Down and I am not recommending you apply these to your collection without first testing it for a few months on replaceable plants. This application worked well for certain plants, which is why I had originally written this post. However, I discovered that different vinegars (white and apple cider), as well as malic acid eventually adversely affected some plants. Vinegar specifically seems like a bad option and I wouldn’t recommend it to lower your pH—it’s too easy to irreversibly damage roots. Concerns also arise about the quality of citric acids out there—I suspect some brands
have more sugar than others and if that’s true, it could lead to an increase in fungal or bacterial issues. Christiopher Coburn deserves a special thank-you for raising this issue.