After playing minor hockey with the Mississauga Reps of the Metro Toronto Hockey League, Tanti began his junior career with the B-level St. Michael's Buzzers; he scored 58 points over 37 games. The following season, Tanti moved up to the major junior level with the Oshawa Generals of the Ontario Hockey League (OHL). Recording 81 goals in his first OHL season, he broke the league's rookie goal-scoring record previously set by Wayne Gretzky (70 goals in 1977–78). He added 69 assists for 150 points over 67 games. In the playoffs, he recorded 15 points over 11 games.