I agree. Its very strange. In amm my reading the only reason I have found for a fish to have its mouth open is if something like a blister inside the mouth is irritating the fish or if the fish was simply born that way.
When euthanizing please avoid flushing the fish or putting it in the freezer. These methods are painful for the fish and are not accepted as humane methods of fish euthanasia by the AVMA.
Some more humane and painless options include:
-A commercial euthanasia aid like Finquel
-Bludgeoning (wrap the fish in a paper towel and give it a quick whack in the skull with a heavy, blunt object)
-OR, since the fish is small enough that freezing will occur fast, you can freeze water so that a thin sheet of ice forms on the surface and then immediately plunge your fish into the frigid water.
Again, please never put a fish in room temperature water before freezing it. It may look like it is 'slowing down and falling asleep' when in fact the process is like full-body frostbite.