my TDS reading is over 800ppm...

hi i have 60g reef new tank with 30g sump.2 clown fish and yellow tang and som cleaning snails atd...
tank was set up from tap water and i just buy RO filter...
before i add any RO water the tank water is over 800pmm TDS.
i want to change water and start changing with RO water-How much water i can change in first time? %%%%? so i don`t kill my good bacteria...

anybody can help with that???
and one more thing---how i can raise PH in water??? now is 7.8-8.0

thank you