Nicolas is better than me to explain this but …
if you boil oak leaves the water is less coloured and pH fall well no ?
Phenol make uprise conductivity but the decomposition of leaves product hydrogen wich make fall the pH and not modify too much the conductivity.
I use to put peat pellets in filter, peat moss in socks and boiled leaves and alder with many ceratopteris cornuta in the tank and i manage to have very low pH around 2.72- 3 with conductivity like 12 to 20 µS/cm.
David
PS : Nicolas au secours !!!!