Growth Technology pH Down is 85% phosphoric acid and needs to be handled with care. If you get it on yourself or your clothes, wash it off immediately. The pH of a nutrient solution is critically important. Plants absorb minerals best between pH 5.5 and pH 6.5. pH adjustment should be the done last after nutrients and additives have been added to the tank. Recirculating systems may need the addition of pH Down every day since the pH of the nutrient solution tends to rise over time.The best pH for a nutrient solution when growing plants hydroponically are between 5.8 up to 6.2. The concentration is 81%, which is very strong so handle with care. Some other products on the market are considerably weaker so check the concentration before you buy. This is a very dense liquid and should feel much heavier than water. Growth Technology pH DOWN contains phosphoric acid (H3PO4).