The term pH Down refers to commercial products or regular household substances that are added to a hydroponic nutrient solution in order to bring down the solution's pH levels, helping in become more acidic. The term is often in the title of commercially available products, or household products can be used instead. Commercial pH Down products generally contain food-grade phosphoric acid. In contrast to pH Down products on the market, there are also pH Up products available. Such products typically contain potassium hydroxide and potassium carbonate; otherwise regular baking soda can be used in a pinch.

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Because pH values fluctuate easily with minimal input, it is advisable to start off with very small amounts of pH Down and continue testing the solution with a pH pen, pH meter, or litmus paper.