Welcome to planted tank
How big is a single bulb nano cube? How many watts is the bulb?
Why do you want to have exact pH of 6.5? Using 100% RO/DI water is not a good idea since it's lack of mineral content. Also it has no buffering capacity. If you have really hard water in your area, you can always mix it 50/50 with tap to replenish the minerals and buffering cap. If you inject CO2, pH will be lowered anyway.