Why Rainbow fish are shaking?

[blaze phaze]

Hey im a new owner of a 55gal tank. It came with 4 fish already and a pleco. I have two rainbow fish about 1or 2years old. The bigger one (assume to be a male) is always chaising arround the smaller on(assume to be a female). They go in a corner face to face and start shaking. I was just wondering why or what are they doing?!?

Sent from my SGH-I747M using Aquarium Advice mobile app