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Most fishes are oriented dorsal-side up through one of two (or possibly both) responses: Dorsal Light Response and Gravitational Response. The fish you point out in the picture belong to a group known as the Upside-Down Catfish (for obvious reasons  ). They respond in a completely different way, know as Ventral Substrate Response. This response system drives their behavior of orienting themselves based on objects or substrates near its ventral surface (underside). This is why the ones on the bottom are dorsal-side up. Now in a free-water situation (no substrate or objects around) they swim completely upside-down. This is due to the physiology of their inner ear, which is more science than you want to know, but to make a long story short, the signal sent to the brain is switched, so that its gravitational response is the opposite of what we would think of as normal. Hope this answers your question, and perhaps others that know more about the specifics can chime in .
Great Answer Dasyatis - I must admit the name upside-down catfish drew and audible "no" from me, but I googled it and...Trouble is we are now gonna have an orientationally challenged sheep on our hands