minimum temperature is 5C. Ideal temps are 10C-20C Axies will still eat at 10C, but may not eat if they temp is lower.
Since we've been having colder weather, 2 of our tanks have been hovering round 12C; our others are currently at 16C. Our axies are happiest when its colder than warmer.
The warmer the tank the less oxygen in the water and can stress your axie as it gets to 24C+