Is there a danger, for plants or fish, by maintaining an inconsistent CO2 level to your tank?

My pH runs about 8.2 if I were to add CO2 am I correct in that providing CO2 may bring the pH down?

Our water is very hard. Calcium builds up on anything in the tank in just a day or two. While I know this is hard on the equipment are there any benefits to having such hard water?

I am just full of questions. Thank you for your time.