Heliox is a breathing gas mixture used in technical and commercial diving. Heliox is composed of Oxygen and Helium only. Having no Nitrogen it avoid nitrogen narcosis that happens in deeper dives.
Trimix is a mixture of three breathable gases. Oxygen, Nitrogen and Helium. It is used in technical diving to reduce the effects of nitrogen narcosis. Because helium has lower density, Trimix is also used to reduce the effort of breathing caused by surrounding pressure at greater depths.