It came right on half time and knocked the stuffing out of their already dispirited League One opponents.
Later the tall defender, newly signed from Cambridge United, picked up a booking but this was a minor blemish and Notts swept to an impressive win.
City played out the final half hour without teenager midfielder Cole Skuse who was sent off for his second yellow card but the truth is they never looked like saving the tie.
County finally did for them when skipper Julien Baudet thumped in a free-kick from 35 yards with the ball going straight in over the head of keeper Steve Phillips.
The League One side showed hardly any urgency but had some early chances with James Quinn lifting a shot over the bar from five yards and Bradley Orr sending a header straight at Kevin Pilkington.
The nearest Notts came to being unsettled came when Pilkington failed to collect the ball on the edge of his area.
City were too slow to react to the sight of an open goal and Tann was able to make his first sizable contribution with a diving header to the safety of the corner flag.
Notts were beginning to appreciate this could be a game for the taking and they took it right on half time.
Orr gave away a free-kick with a mis-judged challenge on the left. Lewis McMahon, who had been struggling with an injury, saw the ball into the six-yard area and through a mass of players Tann appeared to make his scoring debut.