In a large saucepan on low heat, melt butter and marshmallows. Add rice crispy cereal and stir to combine. Press rice crispy treat mixture into an even layer in a greased 9in by 9in baking pan and chill for atleast 5 minutes. (I let it chill while I prep my candy)
To assemble: Cut rice crispy treats into 1in by 1 1/2in rectangles, place 2 julienned slices of sour straw and 1 swedish fish in the center of each treat and wrap with 1 slice of fruit roll-up to secure the swedish fish, repeat with the remaining rice crispy treats.