...but what are you actually teaching the kid about nature by just taking it out of the wild for a bit then just throwing it away ? It also says to the Kid that this is just fine to do, Of which it is not fine to do. In such a case here it would be re-enforcing the same behavior that should be discouraged.
It could teach the kid to be interested in nature by watching a wild fish, possibly nurturing a life-long interest in the natural sciences. There are hordes of kids that never get the chance to look at the natural world (case in point: a college student in a class I taught who ran away screaming from a butterfly during a population study). A lot of kids in my area are barely allowed out of the house or yard, to keep them out of trouble.
It could also be an opportunity to teach about adaptation and not introducing species where they don't belong. We could say that we will watch the fish, but since it isn't adapted to life in our little bowl, we have to let it to. Similarly, if we have a goldfish in another bowl, it isn't adapted to life in the lake, so we shouldn't let it go. Since the goldfish was bred to be in a bowl, it is well adapted to that environment. We can even talk about the importance of not letting the two fish meet so they don't pass "germs" into the lake that don't belong there.
This kind of stuff isn't above a 7-year-old's head. It's amazing what they can incorporate into their understanding of the world. But if the kid never gets the chance to watch a part of nature, he may have no reason to care about it at all. The result of this is seen all the time. When I teach about environmental issues, I am constantly asked "why do we care"? Oftentimes, it has to be couched in terms of cost in dollars ("what would it cost to clean up the water mechanically"). I can only believe that this is the case because the student did not develop an appreciation for altruistic sense about the natural world. And these are college
students, who will be out there making decisions on how to act, what to spend money on, how to vote, and what to teach their own kids.