Maybe this is a good point, maybe not, but granite is extremely inert. Each summer I go canoeing in a region in Ontario, Canada where the local rocks are almost exclusively granite. Compared with lakes here in Pennsylvania, they are relatively lifeless because there are so few nutrients leaching into the water from the granite. Like I said, that might be seen as good or bad. If you have heavy root feeders, you would certainly want to fertilize your substrate.