I also keep my tanks in the garage. I’ve lost a lot of fish over 3 different summers. After the second summer of losses, the next summer I ended up dropping the water level to let the filter output give more aeration, added an air stone, and that seemed to work that 3rd summer. However, this past summer, I lost 2 fish due to the heat, but one of them was a cold water fish, but the other one...even though it was a tropical catfish, it didn’t make it. The Bichirs that it shares the tank with are all still alive.