You might want to test the stability of your ph down/up. I have been looking at this myself since my water ph is > 10 this time of year. Found many substances that will drop the ph, but only 1 that has remained stable. Not exactly a clean solution for hydro either. For that I'll stick to phosphoric acid. It's extremely stable, once ph is set, it stays.
Citrus (including citric acid) I have found is just too unstable. The next day my ph is back above 9. Vinegar is worse. Ph starts to climb back up within 30-60 minutes. Have not tried apple cider or cinnamon yet.
For my outdoor plants, I add 10-15 gum leaves to a 20 litre (5 gal) bucket of water and leave it overnight. Start ph 10.9. End ph 7.1
Extremely stable too. Left a bucket for near 2 weeks and it held the ph nicely.
Might be worth testing different types of leaves. I only use gum leaves because I see about 10,000,000,000 of them while I'm filling the bucket with water.
You can create a more ph down type of substance with a stronger batch. 1/2 fill a bucket with crushed leaves (dry or fresh, doesn't matter) and the remainder with water (maybe use distilled so it doesn't go off as fast). Leave it for 12-24 hours and strain. Leaving it longer I assume will make it more potent. Use the liquid as ph down per required. The strength of it will determine how much you need. I think I needed about 150-200ml per 5 gal with my overnight test liquid. This had to bring the ph down from 10.8-9 to neutral 7.
Urine will drop the ph but I'm not too sure about stability. I was dry reaching testing the ph of my piss water and there was not a hope in hell I was going to leave it 24 hours and test again... Haha!