If you get a high enough CO2 level (depends on KH as well as pH), you don't need to use a pH down product. However:
I'm not sure if you are referring to one particular product, so I'll talk about reducing pH in general:
Some pH down products use phosphoric acid (H3PO4) or monobasic sodium phosphate (NaH2PO4). Both are acidic (pH<7) and will lower your pH. The problem is the resulting phosphate available for algae growth. The acid reacts with bicarbonate and carbonate ions in the water, ultimately producing water, CO2, and phosphate.
Other pH down products use a dilute mineral acid solution (HCl or H2SO4). The resulting chloride and sulfate ions will not cause a problem in the tank. Here the acid also reacts with bicarbonate and carbonate ions, producing water, CO2, and the chloride or sulfate ions.
One other warning though: these products work by destroying some of your bicarbonate buffering (KH). If you are injecting CO2 as well, you could end up with a pH crash. Also, be sure to re-measure KH after using these products BEFORE you turn the CO2 on.
72g bowfront planted, CO2, 4x - T5HO, Eheim 2213 and 2217, 2 angels, pristella tetras, blue tetras, betta, albino bristlenose pleco, albino cories. Sword, vals, hygros, ludwigias, java moss and fern, anubias
2g Mac-quarium. Clown gravel, fluorescent plastic plants, and 2 guppies.