Okay, since you've only had him a short amount of time he could still be adjusting to his new surroundings. Often it takes them quite a while to really settle in.
My boy in my avatar, Remmy, he took a whole month to decide he wanted to eat and feel at home so don't worry, just keep his tank clean, offer food and he should be alright
Keeping the tank dark as you've been doing is good too, but he just might take a little more time to adjust and be happy again. In the mean time, just try to make him as comfy as you can, offer food when you can but if he eat's once a day that's great, if he doesn't then you can offer again some time later.
Also any reason the tank is at 82? Do you have an adjustable heater or is it just the warmth of summer?