That PH change will most definitely affect your fish and I would like to say that one of my biggest peeves since coming to this forum is that people are told that if your going to keep discus you have to do big water changes, and I hope someone picks up on the point I'm trying to make here the big water change people I think sometimes forget to explain their abilities and Knowledge they have that enables them to do these changes without destroying their water parameters mainly ph I think it's second nature to them I myself do not have the ability or the knowledge so I choose to do smaller more frequent changes while monitoring my parameters mainly amonium and nitrates I would say to ease yourself into the big changes and start by doing the biggest change you can without altering your ph and then speak to a big changer and gain some knowledge from them