CO2 isn't going to allow you to run higher temps in your reservoir. 65-75 is about the range you want in your tank, room temps can run a lot higher though- 90s probably OK with CO2. In your tank you wanna keep it cool, cold water can hold more oxygen than hot water, plus as you creep higher you run the risk of different pathogens starting to grow. I freeze water bottles and use them to keep my tank cool. You can also use reflective covers to help keep your tank from absorbing as much heat.