I keep all my US natives ( atm just the gars) at "House temp" Ie if it's a comfortable temp for you it's good for the gar/native. always exceptions to the rule but never had an issue keeping my gar in the 60's up threw the mid 80's... richard or Sol might have further imput but I look at it also being an air breather tank temp/air temp being similar temps is prolly not a bad thing for these guys. avoiding temp swings and the usual "swings" in perameter is what I focus on more so then an "exact" number. I do have a heater in the tank set to the mid-70s, long story short.. we just moved and the fish are now in an unfinished basement so I'm wary of major temp fluxes.. but as soon as I am comfortable and know the average temp swings down there I will likely drop it further. My young cuban also shares the tank and I'm not comfortable letting it drop to low for him. If it was just my 2 floridas ( both 1yr+) I would probablly still run the heater in the tank set to approx 68 and go from there.
PS- I always avoid glass and non-submersible heaters.