You can easily make your own PH down solution for much cheaper than you can buy in the store. You can make a gallon for around a dollar. BE SURE TO FOLLOW THE DIRECTIONS CAREFULLY TO AVOID INJURY WHENEVER DEALING WITH CHEMICALS
Note: with all of that being said, sulfuric acid that you get as “battery acid” is considered safe to deal with, just don’t be stupid. I know that at first it can sound scary or off-putting at first to think about dealing with sulfuric acid, however it’s actually pretty benign in this form, and is also beneficial to your plants.
Buy some H₂SO₄ aka sulfuric acid at your local auto parts store - it is around for 7 or 8 dollars, and is enough sulfuric acid to last you several lifetimes. They don’t usually keep it on the shelves, so you will have to ask them for some “battery acid.” The sulfuric acid you will buy at the auto parts store is most likely 33%, which means its already diluted. Pre-diluted sulfuric acid is marked as battery acid/fluid meaning it has already been diluted. However, we are going to dilute it even more to around 1.65%.
Pour off 2 cups from a gallon jug of distilled water - DO NOT USE TAP WATER - IT IS UNSAFE TO MIX WITH SULFURIC ACID.
Put on some safety glasses and some rubber gloves and pour 1 cup of battery acid in to your jug of distilled water with a funnel and let it sit for 5 minutes to naturally mix, and then put the top back on the
gallon jug and gently shake to mix. If you get any on yourself be sure to wash it off.
Never pour water into an acid - always pour acid into water. IF YOU POUR WATER INTO ACID IT MAY BOIL OVER AND BECOME UNSTABLE
Some people like to add one or two drops of natural blue food coloring to their homemade PH down solution so nobody will drink it accidentally.
CLEARLY LABEL IT AS POISON AND KEEP IT STORED PROPERLY.
Sulfuric acid is relatively safe to handle, as long as you take precautions, and don’t drink it. Mechanics and wine makers use H₂SO₄ it all the time, and it is safe to use. Phosphoric acid is derived from sulfuric acid. Orchards and berry gardeners and potato farmers commonly use sulfuric acid.
Sulfuric acid provides sulfur in the form of sulfate, which is essential to plants and is often low in available water sources or fertilizers. High rates of sulfuric acid do not cause any issues with crops.
As mentioned, DO NOT USE TAP WATER - tap water may contain trace amounts of copper, which when mixed with sulfuric acid creates sulphur dioxide which is a highly toxic gas.
As with any other PH down, you wil have to test it yourself with your water and nutrient solution to see how much you need to lower your PH to the proper level.
If you have a remote hydroponic reservoir you can pour the 33% battery acid straight from the bottle into the reservoir without pre-mixing it in water.
DO NOT INGEST - EVEN AT DILUTED STRENGTH IT CAN CAUSE BODILY HARM IF USED IMPROPERLY.