Adam Rooney had headed Chesterfield into a 13th minute lead, but Nix curled in a left-foot shot in the 77th minute to deny the home side what would have been their first victory at Saltergate since September.
The conditions were awful at times with pools of water slowing and stopping the ball, but in the end referee Anthony Taylor was proved right to let the game go ahead.
The rain finally stopped early in the second half and conditions improved slightly although the saturated surface was always a test for the players.
But Mr Taylor showed excellent judgement and took the conditions into account to produce a game which provided some good entertainment for the fans on a thoroughly unpleasant afternoon.
Chesterfield were well worth their first-half lead given to them by Rooney's header from a Jamie Ward corner, although centre-half Janos Kovacs collected his fifth yellow card of the season when he slid into Nix.
Bradford were better after the break and Joe Colbeck was unlucky when a 25-yard shot struck the underside of the bar, but Jamie Lowry should have added a second for the home side, but he fired over from close range.
Bradford's switch to a 4-4-2 formation proved inspired when Omar Daley played a part in the equaliser when he set up Nix, who sent a left-foot shot from just inside the area past Barry Roche and inside his right hand post.
It was an excellent strike but Chesterfield almost regained the lead five minutes from time, when the dangerous Ward turned on the edge of the box and drove in a shot which Donovan Ricketts had to push behind for a corner as he dived to his left.