The leaves won't help you lowering the GH/KH significantly.
It;s more because of the anti fungal properties etc.
If you really need to lower this, best is to boil water or use reverse osmosis.
There are many "recipes" for black water with different ingredients.
I tried using alder props, oak leaves, rooibos tea and catappa leaves.
The thing i like best are the leaves, mainly because my tads will be able to nibble on them as well.
Since there are back up studies providing evidence of the healing properties of the indian almond leaves i now mainly use these.
You may boil water and drop in any of these ingredients, sift it and you got yourself black water.
This can be stored in bottles and mixed with tap water for usage.
I don't know the quality of the tapwater in the US, but in Holland tap water has an even stricter protocol then bottled water,
so i just take the calcium for granted.
Used RO water for a while, but it gives a lot of "waste" water, so i now descided to just clean the calcium of the windows more often

Otherwise you may take some leaves and put them in stockings, just put them in your cannisters while they are ventilated.
It will give you a similair effect.
The black water is easier in use when you tap you're water or use an osmosis system,
you just have to put in the black water and you're ready to go.
Putting in some crumbled leaves in stockings will take a while to draw the tea from.
Just as in regular tea, boiling water draws a lot quicker.
Don't know if it is a proper saying in English, but as we say,
there are many roads leading to Rome
