That's the rainbow shrimp! It can also see way more colors than humans and break its own aquarium if kept in one.
Blue glaucus, or blue sea slugs, aren't fish but they look like fish, so a quick fun fact - they actually float and swim upside down. Their bellies have the cute blue stripes, but because of the location of their gas sac, it faces upwards to blend in with the water, and their silver backs look like sunlight reflecting from the surface.