Most fish have developed methods to not become dead prey while at periods of rest. Some actually tightly lodge themselves in rock gaps. Some develop relationships with other life forms, such as anemone fish with anemone. It is not normal for fish who are normally prey to just lie about in the open. Sure alot of fish are tank raised and some have probably never been actively preyed upon. But, there is a difference between laying lifelessly on the bottom of a tank and sleeping in a secure location like a cup. As well there is a huge difference between a noticeable increase in gill action and water swallowing of a fish not receiving enough oxygen and the "breathing" patterns of a fish who is "breathing" from well oxygenated water. A lot of the behaviors we see in fish are mistaken for normal and/or acceptable behaviors when they are not. Low oxygen behavoirs at night are one area that aquarists have ignored symptoms for years thinking them normal or acceptable. A lot of these "normal" and "cute" behaviours disappear when night time oxygen levels are brought up to near day time levels. There is no where near the difference, percentage wise, between day and night time dissolved oxygen levels on the ocean reefs. Also a lot of home ownwers actually run hotter room temperatures at night, which equates to hotter tank water temperatures. The hotter a tanks water the lower amount of oxygen it will hold. Also the higher the temperature the more rapid the break down of fish waste and dteritus to ammonia. Fish have obvious "breathing" problems in the presence of ammonia. Plus the hooter the temp of the water the higher the oxygen demand by the tanks bio system. Plus, plus, plus . . .