I have put Oak leaves in my shrimp tanks. I didn't check the ph to see if that was affected. But the reason Oak leaves are suggested for shrimp tanks is because the shrimp feed on the microorganisms that feed on the leaves. When using leaves. Collect the dry Oak leaves in an area where they would be free from pollutants. Before adding them to your tank soak them in tank water for a few days so they sink and a lot of the tannins will leach out. Soaking the leaves first will prevent your tank water from turning brown. Oak leaves break down slowly so they will last a long time in the aquarium.