PH Monitor

[Jaws]

I was wondering if anyone local had a PH monitor I could borrow for a short while? I have a Hagen test kit but the colour keeps turning black and i'm pretty sure black is not an option on the PH meter. My tank has developed a brown algae on the sand and on some of the corals as well since this morning. I did a 20% water change yesterday and just used tap water with a dechlorination reagent. Could this be the cause of the algae? I tested all the parameters and they all seemed fine.