If I read right that you are not doing water changes, that is likely at least 50% of the pH issue if not the whole problem. Built up waste decomposing in the gravel creates acidic compounds as byproducts. Without water changes and a gravel vac to remove both the compounds and the waste producing them, it will probably be impossible to maintain a stable pH especially if you are trying to go with softer water. I have repeated this over and over in my tanks, I have very soft water with little buffer, but the pH remains stable as long as I'm doing my weekly 25% water changes. Leave it for two or three weeks or more, and the pH goes through the floor. Regular water changes are really necessary for good fish and plant health, not just for pH reasons but for a multitude of other reasons (one other big one being the constantly increasing TDS level as the water evaporates).
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