[Fish]

Let me throw another one out there, just for grins. I'm no expert in chemistry, but this kind of world has some interesting RPG possibilities:

High-chlorine atmosphere. Chlorine has a lot of interesting uses in manufacturing, including production of polycarbonates and polymers. It is also toxic to a great many living things, due to its tendency to bond with water to form hydrochloric acid — chlorine is a great disinfectant. Might not a planet with a chlorinated atmosphere be useful in making plastics, and other products that require food-grade antibacterial sterility? Chlorine is denser than Earth-type atmosphere, so it would tend to collect at lower elevations; perhaps chlorine would concentrate in caverns and valleys, and there would be a more breathable atmosphere in the hills and mountains.