I have very hard tap water. The pH out of the tap is 8 or 8.2, and the general hardness (it's been awhile since I did hardness tests on straight tap water, so I can't quite remember) is about 13 dh.
I get
RO water from a machine in my grocery store. I don't check its pH every time I get it, but recent readings have been 6.6. I have two 5.5 gallon tanks, so it's not a big hassle to get a gallon or two of
RO water a week.
I mix half tap water and half
RO water. I don't add any additives. I did buy one bottle of
RO Right, but I took it back. It only added one dh to the water, and I figured I didn't need it since i mixed with tap water. The general hardness (of the mixed waters) is 5 dh and pH 7.2, and 6 dh and pH 7.4 (two tanks).
I have betta tanks, and it's been working out well for them. I did pH tests frequently when I first started doing this, and I don't get any pH swings. The bettas seem happy and healthy.