Reverse osmosis questions

Hey, I'm moving soon and just found out my tap water has a PH of 9.25. I currently have south american fish ( Arowana, Lima cat, etc.) I'm not for sure but that ph seems too high for my fish. The house already has a culligans reverse osmosis unit.

I know nothing about RO units. Can someone fill me in on how reverse osmosis systems work and if I should use it? Also do i need to add minerals back to the water as well and if so can someone explain how to do that?

Thanks in advance.

When you say mix. Mix tap water with RO water and then find the parameters that I want?

One more question I just checked my KH and GH. Kh 4-5 and GH 5-6. Will mixing my tap water with RO water lower my KH too low? Ive read that you always want your KH above 4?