With radio controlled gills, they turn, and move through the air as if they were swimming. You can buy the clown fish, shark, Piranha, Regal Tang, and they are filled with helium through a one-way valve. You will need to buy a can of helium, or have a full tank handy to inflate the fish.
There are also Angry Birds Air Swimmers.
Here is an instructional video on how to setup and fly your RC fish:
*The toy itself ranges in price from $50-$100, depending on the seller. You can also buy a 2 fish combo for around $110-$120.
*The Air Swimmer helium tank sells for about $43.
*Replacement balloons, (in case you tear a hole in yours) are sold separately for about $25.