You could try the ph test strips, they are really cheap. Mine measure from like 6.0 to 7.6. The only problem with checking the run-off ph is that your water will be tinted, and could throw off your results. I ran my run-off through a paper towel (or clean towel would probably work too) and it cleared it up. Not sure if it affected the ph much, but the results were nearly similar before and after I cleared the water.