water parameter levels

I have a lightly stocked 20 gallon with 4 plants, but my ph always reads 8.0-8.5. ammonia and nitrites 0ppm, nitrates 0ppm? i mainly want to lower ph to at least 7.5 and have a bit more nitrates. i know plants take in nitrates but im not sure how to achieve my parameters without using buffers or a lot of products, any advice would be really helpful. ive tried distilled water top offs also.