I need water help.

Hi! I'm new to fish and I have only one tank, a 90 gallon, with one
tiger oscar and a pleco. (I'm hoping to add a pictus catfish to eat
the scraps from the oscar as soon as my local fish store can get me
one that's big enough to not be lunch.) My problem is my tap water.
Quite often, and without notice, my city changes the water. For a few
months, the tap water had a ph of 5 and suddenly it changed to 8.5.
This makes tank cleaning too adventurous for me. I'm wondering if a
RO unit would be a smart move. Could someone please give me some
advice? Thank you!

Ann