So to lower kh... Peat or black water extract? I dont want to lower my ph though
I said what you can do to lower your kH. It doesn't mean you should do it. The kH range I mentioned is what I consider best for plants based on my own observation. It also doesn't mean your plants cannot grow in kH 8
My plants grew fine in higher kH and gH than yours. Some of them like Vallisneria use kH as a source of Carbon and some do not like it at all as it will limit CO2. Tonina on the other hand prefers soft water with lots of CO2.
There's no short answer to all plants. You will need at some point to experiment and learn which are the values that yield the expected results.