Tropical Fish tank Co2

I have a tropical 40 Litre tank which has had its PH level stable for 2
months now.  I have placed live plants in the tank and was have
purchased a (not too expensive) co2 kit.

I know the co2 can alter the water chemisrty and was wondering the
ideal balance between PH and CH levels or what to watch out for when
introducing co2 into the tank as to not suffocate/stress out my fish.

Any info would be appreciated.

Thanks