How to achieve ph < 6.0

Previously, I was using ada v2 for just about a year. My ph was about 6.0 (yellow on api test kit) and I noticed recently that my ph is now 6.6. I replaced the soil recently with more ada v2 and I can’t seem to get the ph to drop. Using RO and remineralizer to a gh around 4-5

I thought maybe the Ada v2 is no good. Bought some new soil called 2HR aquarist feast. I noticed my ph drop but then I had a huge ammonia spike. I took out the soil and put it in a bucket for now to let the ammonia leach. I noticed my ph dropped significantly in my tank as well, reading 6.0 again even after removing new soil. Now it’s stabilized back at 6.6. I checked the ph of my bucket water, ph is also 6.6…. So I am curious am I doing something wrong? The Ada v2 is still in my tank, and neither soils seem to be buffering to the desired ph