Review the original item:
My cat Buster is always tipping over the garbage can
then he searches through the trash for milk jug caps, which he bats
around the floor, the feline version of playing ice hockey.
You chose the correct way to fix the error:
My cat Buster is always tipping over the garbage can,
and then he searches through the trash for milk jug caps, which he
bats around the floor, the feline version of playing ice hockey.
My cat Buster is always tipping over the garbage can is a main clause. Then he searches through the trash for milk jug caps is the second main clause.
A comma with a coordinating conjunction like and is a great way to connect the two main clauses that create the fused sentence.