I always have ammonia binder/water conditioner on hand. As a rule of thumb, I would keep enough on hand to treat the entire pond twice. I like to use a conditioner that also treats for chloramines. Our water now (we're on well water currently) but I still prefer to use one that could treat chloramines. I use Safe by Seachem.
I would get the binder, add enough for the reading you're getting and see how they do.
Within this thread essentially all of the bases are covered.
I have suggested how to deal with the potential water issue regarding ammonia or chloramines, Stephen advised to make sure that you remove parasites as a possibility and to s/s thoroughly and Richtoybox has counseled you on how to deal with the pH concerns.