add any acid as you already do for the water that goes into the pot.. does not have to be overpriced "ph-down" branded product. just use 5% vinegar you likely have in your house already.
If you try to balance it, remmber it is a log scale. each whole integer from 7 is 10x more than previous ratio of h30+:oh-. if the 'middle' is skewed just under 7, the low side is significantly more hevily weighted thant he high side in thsi context... don't go as low as it is high. get a calculator out if you have to. log // -log will help choose an intelligent counterbalance pH.
your nutes are not buffered, so you are experiencing pH-drift. Find nutes that are ph-buffered and this won't be a concern. won't need that ph down or up, either. My pH strips collect dustt for years before i use 100 of them, lol. better made soilless nutes = easy button