Thank you All for reply.
Fiftymeatballs - as you know pH swings a little here in NYC.
So from what I tested maybe a year ago, I had 7.1pH tap, and 6.8pH after 24 hrs, and then back to 7.0-6.9pH.
For my cichlids I buffer the the water, and it works. I get 8.6pH.
To keep the low pH in my upcoming betta tanks, I'm thinking to use RO/DI device, but not for 100% water. Also I have some Almond tree leaves.
Was hopping to collect some branches from park and oak leaves and cook some black water. Do you have any other ideas?
Oh yea I know exactly what your water is like, we have the same source.
Why do you think you need RO?
RO water is stripped completely and is very susceptible to ph swings, you have to remineralize. Seems you know this since you say you plan on only using some RO water and some tap. Honestly I think it's a waste of time and money for keeping betas given how good our tap is. If you go RO keep in mind our tap water mixed in won't be enough KH to buffer your PH. You will need to dose baking soda carefully and accurately, or keep a limestone rock in the tank or crushed coral in the filter. Achieving a KH or 1-2 is plenty to buffer and is very important for stability. I dose a GH booster to our tap for the plants as well.
We have perfectly fine tap water for keeping most all species of fish and plants, I recommend you add a little KH and GH based on species since its very similar to RO water. Most people would kill for our tap water. The ph is neutral to slightly acidic, KH 0-1, GH-0-1, TDS-40-60.
You will remineralize to the liking of your fish and plants, lower PH with peat, driftwood or aqua soil. You can up the PH with crushed coral like your doing etc. It's easy to work with. You could get away with just tap without remineralizing if your carful with what you put in the tank hardscape wise. Driftwood can lower the ph so much it crashes. So if your planning on lowering a already volatile PH water like our tap just keep an eye on things. I have experienced ph crashes in both my discus and shrimp tank before. Large driftwood in my discus tank caused the ph swing to drop into the 4's killing all the bacteria in my filter. The aqua soil in my shrimp tank did not hold a steady buffer and in combo with RO water and tap my ph dropped to the 4's killing bacteria and some shrimp. The shrimp like 0KH which is why I did not add any. Now I am buffering at 1kh, holding stable.
If you want to spend the money on RO just make sure you add back KH and GH to the water. Be very carful adding KH if your dosing baking soda, a little goes a long way and you can kill your beta with unstable parameters.