Don't take this as factual as but i have found several people say sulfuric acid can be used, its in ph down at the local pet shop. I'm currently using it with no problems. Cost me $20 but well worth it. I was dealing with the same ph as you, but have now got my clones down to 6.8 safely and plan on getting them down to 6.0 eventually. My plants in soil didn't really like the 7.5 tap water either, probably why i've had yellow leaves mid way on the plant.