So, this is what I think is going on. My substrate is a mix of 65% fluorite, 30% Azoo plant bed and 5% onyx sand. The KH from the tap is around 5dKH and after a week it is around 1dKH in the tank. Should I increase the number of water changes (at the very least stop waiting more then a week) and remove less water to reduce the fluctuating KH when I change the water? Basically, it changes from 1dKH to 3dKH after a 50% water change.