I was browsing the web and seen this question so I thought I would do some looking around and this is what I found.
To understand why fish yawn you first must know how fish breath. We all know that fish have gills but do you really know how they work? Water only holds 5% of the oxygen that air does and is 800 times denser than air so fish must have a very efficient means of getting oxygen and using energy. Most fish have a buccal cavity which they use to pump water across their gills. As the water flows over the many blood-filled gill filaments, CO2 is released and O2 is absorbed all by simple diffusion through the gill walls.
The gill filaments actually strain the water so they will acquire small particulate matter in them too. When a fish "yawns" it is forcing water to backwash across the gills cleaning the trash off that is blocking the filaments. So now you know. When you see them yawning they are just doing a little housekeeping. The more you understand your fish and its environment the better chances you have of keeping them for a long happy fish life.
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