This conflation was uttered by Al Sussman in his Beatles podcast, “Things We Said Today.” It is a mashup of “opens up a can of worms” (an array of difficulties) and “a whole other kettle of fish” (to be entirely different in scope or description than what was just being discussed). Certainly fish and worms are associated and probably led to the mixup here. And then there are the two containers, kettle and can. The expression “a can of worms” gets bollocksed up often. Some examples posted on this website are “I don’t want to open up that hill of worms” https://malaphors.com/2016/04/25/i-dont-want-to-open-up-that-hill-of-worms/, “That’s a real ball of worms” https://malaphors.com/2012/12/18/thats-a-real-ball-of-worms/, and “That’ll be a kettle of worms” https://malaphors.com/2013/10/12/thatll-be-a-kettle-of-worms/. A big thanks to John Polk from ClichesGoneWild for hearing this one and passing it on!