Joe Exotic Calls Atlanta's Federal Prison the 'Bottom of Hell'

Joe Exotic's had enough of his living conditions behind bars ... so he's putting the whole place on blast, basically saying Satan is his warden now.

The "Tiger King" star fired off on Instagram Thursday -- yes, while in a federal pen -- and wrote, "I have officially landed at the bottom of hell, at the Federal Prison in Atlanta GA."

He continued "Senator Ossoff, Warnock and Walker are all lying to you Black Voters of Georgia because the Animals at the Atlanta Zoo are living better then [sic] your loved ones are in here I promise. #fixthiss***".

As you know ... the documentary star is serving a 21-year sentence for trying to hire 2 men to murder his nemesis, Carole Baskin.

Even before his transfer to ATL -- he was previously in a Texas federal prison -- Joe's had a rough go of it behind bars ... battling an aggressive cancer. In fact, he's moved a few times in order to get better medical attention.

The Atlanta facility he's in has reportedly been at the center of a monthslong investigation over allegations of drug abuse, violence and unsanitary conditions.