For plants to grow their best, it's important that the nutrient pH be slightly acidic. A range between 5.5 and 6.5 is considered ideal for most crops. General Hydroponics' pH Down Concentrate is made with phosphoric acid. There are many pH down products on the market, but you get more bang for your buck with GH's pH Down. Use sparingly, a little goes a long way.
General Hydroponics pH Down is available in quart and gallon containers.