Vinegar in water, as I've already stated in your other post.
Leave your soil alone. 7.0 is fine. Adjust your water to a PH of 6.5 and you are golden.
Any type of edible acid will lower the water's PH - lemon juice, lime juice, vinegar, etc. Or try at the pet store and they may have some PH lowering solution.
Take it from someone that taught a Hort. Sci. class - a pH of 7 for cannabis is not ideal if you use soil. Cannabis, as well as a host of beneficial organisms in the soil, likes slightly acidic soil, no lower than ph 6.2 and generally no higher than 6.8. Anything higher or lower in soil will start to cause nutrient lockout. For hydro, I've had plenty of good results with keeping the pH between 6.0 and 5.5.