I'm confused......are you using ph down and cant put enough in to get to 5.5-5.8? Distilled water isn't a great idea unless your rich and don't mind paying that much for water that has no real use in a garden.....maybe an r.o machine. Sure glad I have a well, free nutrient rich water LOL.So the lowest I'm able to get the ph of the water in my res is 6.0, I use tap water that I de-chlorinate and let sit for 24hrs, is the tap water the reason? If so I need to invest in distiller water I believe