Worlds Collide: Part 3 - S3-E6
Plot hole: In their previous encounters with the Triceratons, the Turtles had to learn the hard way that the Triceratons breathe not oxygen but a combination of nitrogen and sulfur, which is detrimental to most Earth lifeforms, and thus had to wear special rebreathers. When the Turtles, Splinter, April and Casey enter Zanramon's throne room along with Traximus, none of them wear any breathing equipment to protect themselves from the hostile atmosphere.
Answer: Per Wikipedia, the series originally aired in the US on Fox for its first six seasons from February 2003 to October 2007. However, the main storyline of the series ended after season five. Following this, the show was retooled with a new art style, comedic tone, as well as a new storyline. The show moved to the CW for its final season and aired from September 2008 to March 2010. However, it would not officially end until the broadcast of the TV movie Turtles Forever, which was produced in celebration of its 25th anniversary, and served as a series finale for the show.
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