Neuroscientists aren’t exactly sure why tank users often see visual hallucinations but speculate it’s due to the brain essentially having nothing else to do. The human brain has evolved to constantly perceive and interpret external stimuli for one purpose–to survive. Over millions of years of development, the brain is now successfully programmed to accept huge quantities of sensory information. In fact, the brain is so accustomed to being bombarded with sensory stimuli that when it is deprived of stimuli, it simply creates its own in the form of hallucinations.
As soon as a flotation tank user leaves the tank, hallucinations of any kind–visual, auditory, olfactory–vanish. There has never been a report of a regular tank user experiencing hallucinations outside of the tank.