Air Fish
What would fish look like if they swam in the air instead of in the water? It's not such a crazy idea -- rays after all, sting rays, manta rays, etc., are fish that are really bird-like in terms of their body style. They really fly in the water. In terms of fluid dynamics, air is a fluid medium -- one that is less dense than water, but that behaves essentially the same. So air fish would probably be generally shaped similarly to water fish. As similar as a ray is to a bird anyway.
Most fish have neutral buoyancy by means of an air bladder; a gas filled internal organ, towards the top of their bodies that keeps them upright and provides just enough buoyancy to counteract their weight -- giving them neutral buoyancy and allowing them to float. This could work in the air, if the bladder were filled with helium for example, or another "lighter than air" gas. Air fish could have neutral buoyancy and float in the air just like fish do in the water.
Air being less dense than water however, their fins would probably need to be larger to give air fish the same degree of maneuverability that water fish have. I have a hard time picturing these air fish darting away as quickly as coral reef fish can, but I'm sure evolution would have seen to that. What I cannot figure out is whether air fish would tend to be larger or smaller than water fish. Granted, water fish come in all sizes; from the neon tetra to the whale shark -- really to blue whales, but of course whales are not technically fish. They share the same general body style, just with flukes instead of fins. Anyway, would air fish tend to be smaller because of the limited lift to weight ratio of helium and other not-so-flammable lighter than air gasses? Or would they tend to be larger? Or would there be a great degree of size variation just like the fish in the sea?
Hmmm…