Are you sure hes in the tank.
They are known to be great escape artists and unless you have a tight fitting lid and no holes, you may find him on the floor!
I had this happen a few times and though I found him all dried up and covered in carpet fluff and hair, floating him in water again he soon came back to life.
If he's definately not gotten out of the tank, then they are also good burrowers, and he may be lying under the gravel, or stuck under logs, rocks etc.