i have never seen fish mate. i had guppies once and i saw the fry...but anyway,
my rasbora harlequins were doing a sort of 'hip bump' and then a 'wiggle'. were they mating? or dying? so they lay eggs? will my shrimp eat the eggs?
IT'S SPRING BREAK!!!!!!!!! WOOOOOOOOOOOOOHOOOOOOOOO!!!!!!!!!
Harlequin rasbora's are egg layers.. but htey do so in a rather unusual way. They invert themselves and lay their eggs on the udersides of overhanging leaves. The behavior you are describing is mostly like a dominance display between two males.