i'd also look into the homeopathic remedy fragaria vesca.
buy 6X or even better 3C strength, nothing higher is necessary. one pellet per day for 1-2 months until tartar is significantly reduced, then as a preventive against new tartar build up, one pellet per week for maintenance.
it might take a little while for the effect to become apparent, but it's a very good, and most importantly all natural, harmless remedy.
oh and definitely stop the wal mart food and feed a better diet!