"The first surreal sensations I had were that I was moving around in the tank, floating right to left, left to right, until I realised I was completely stationary"
When I did it the tank operator told me this happens because there are certain chemicals being released from your adrenal system, and that the direction you feel like you're spinning corresponds to the chemical being released. Not sure if that particular claim is true or not, but apparently there is a reasonably large body of academic research into the physiological effects/benefits of floating.
Also, Joe Rogan's video on the tank is a must watch: