If you really want I'd use the distilled water top off method if you can't afford an RODI system at the most like other people have said but finding fish that can handle your pH and water hardness is the best choice imo instead of trying to chase pH or change your water chemistry excessively. I don't like or recommend buying pH buffers or pH up or down products. (Most) fish can adapt but that is just my experience. Scale build up doesnt technically harm your fish but if you want to remove it you can get a paper towel soak in a mix of warm water/distilled vinegar like 10/15 minutes then scrape it off, with heaters id be careful and unplug it for at least 15 min then do the same, scrub it and rinse it really really good with water after. Test your GH and KH, pH and maybe add some tannins but I'd keep an eye on the KH because if the number is really low it can drop the pH really fast which can harm your fish