Chuck Potthast 's son apparently found his body ... and he told the 911 operator his dad was bleeding heavily -- and didn't look alive.

We've obtained the audio of the 911 call ... which begins with a man identifying himself as Chuck's son -- and telling the 911 operator his father's lying in the bathtub and bleeding from the chest. TLC

Potthast's son says he doesn't know what happened to his father ... and, when the operator asks him if he's awake, he responds by saying he doesn't think Chuck even looks alive.

The operator asks Potthast's son to turn him on his side ... and he responds by saying he'll try -- before noting his dad is lying face down in the tub.

As you know ... Elizabeth Castravet -- Potthast's daughter who also appeared on '90 Day Fiancé' -- announced her fat

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