Actually, if you are using RO/DI water, I wouldn't add any Blackwater extract or soaked leaves until you up the GH a little bit. If you are using RO/DI water, get a decent Calcium/Magnesium supplement and raise the GH to 3-5 DKH or whatever you want.
You can use soaked leaves, which is basically tannic acid to overcome your alkalinity a bit and make tap water a bit more acidic and reduce the pH, but you don't need to soften RO/DI water.
You probably already know this, but just in case... by adding tannic acid to your tap water you do not 'soften' the water, you just overcome the potential alkalinity and your pH drops. The components in your tap are still there, just overcome. All you really do is change your pH and that can sometimes be unpredictable by introducing acid... in other words, your pH can go back to normal sometimes within an hour.
Your best bet overall is to use RO/DI water and a good GH booster and put it where you want it to be.
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