Just making sure, you are getting the pH 6.0 reading after adding T. catappa.
First check your method of checking pH or your RO filter the water should be right around pH 7.0 at 25C (77F)
, temperature is the only thing that will affect the pH of pure water. (For the record the disassociation constant remains the same even after adding “stuff.”)
- At 20C (68F), pure water will be pH 7.08.
- At 30C (86F), pure water will be pH 6.92.
- For pure water, the range is pH 7.47 to pH 6.14 (just about to freeze to just about to boil).
You will add something the water to get to pH 4.5 to pH 5.5. You can pretend you are not adding “chemic acid” when you add T. catappa leaves, but you are adding tannins and humic acid.
Are you using the dried leaves or an extract?
You may simply need to wait or increase the amount of T. catappa. If you are using the leaves you may wish to make your own extract, a version of “blackwater extract,” perhaps adding in peat moss (Sphagnum spp.).
Even though it will tend to resist lowering the pH, I recommend adding 8-20-ppm KH (.5-1-dKH) for stability.
I also recommend staying above pH 5, closer to pH 5.4 for stability.