If only temporary, (a week or two) lemon juice or white vinegar. But for long-term, you'll want something botanically sound like PhDown by General Hydroponics.
It's made from phosphoric acid (much safer than it sounds) and my plants love the stuff. 1/4 tsp:gallon brings my 8.0 water down to about 6.8...and one gallon of the stuff lasts me over two years.