It took quite a bit of google searching to find out how to turn tape compression on and off using the Linux mt command. The mt man page information for doing this is not helpful at all.
To turn compression off use 0 as the switch, to turn it on use 2 as the switch and to find out whether it is on or off use 1 as the switch.
Your commands will look like this
mt -f /dev/st0 datcompress 1 # tell if compression on or off
mt -f /dev/st0 datcompress 0 # turns off compression
mt -f /dev/st0 datcompress 2 #turns on compression