I use a Hanna 98129, it does PH and PPM. Really nice meter but pricey, about $150. But you can get one that will work on Ebay for about $11 and up and they do work.
https://www.ebay.com/sch/i.html?_from=R40&_trksid=p2060353.m570.l1313.TR12.TRC2.A0.H0.Xph+meter.TRS0&_nkw=ph+meter&_sacat=0...