Pellet stove ~20ft from my reef. Even a well sealed wood stove will add a little Co2 to the air, can lower pH a bit, but it's never been enough to worry about.
Temp, on the other hand... yeah. If I'm running the pellet stove enough to have some impact on my furnace's energy use, it's close to 80 in the living room. Keep it that way all day, and the tank temp gets way too high. Impact will depend on many factors, starting with the volume of water in your system. My 60 cube would warm up fairly quick. I'm expecting that the 140g system I'm building will take a bit more energy to push the temp up.