First and foremost, take a deep breath and relax. It's going to be okay. You dont say where you live, but maybe your town is like the one i just moved to where as they state:
Originally Posted by water report
sodium carbonate and carbon dioxide to
adjust the water’s pH and buffering capacity. This change has made the
water less corrosive, thereby reducing the leaching of lead into drinking
The thing is the water is very very soft (Low GH/KH) and thus after the water sits for a day or two it quickly drops in Ph. I added some to my new tank and it was 8.x and after 2 days its down to 7.x. So if you put some in a bucket with a power head and let it run prior to the water change you should be fine. (i'm not even sure the Power head is needed).
Your water changes on a 75g wont be huge and will allow the new water to not make the tank fluxuate so i wouldnt be very worried.
I've always avoided chemicals to bring the Ph levels into different ranges, i'm sure you will find this is the common recomendation. Also, plants and fish will do fine in mid 7 Ph water, i'm not too sure about discus, but others will chime in.
Have a great stay.