No...must resist...will not offer my two cents....no no no nono......*sigh* okay, I give up.
I agree with everyone who said that once you purchase a fish, you must accept reponsibility for the -whole- fish for the duration of its life. The same is true with puppies, kittens, hamsters, children etc.
The problem is, not very many of these creature live long enough in captive care for people to know how long lived and large certain fish get. It is the classic tale of the ID shark. Nobody in the public knows an ID shark can grow to three feet. Nobody save the dedicated aquatic fanatics have had an ID shark live long enough to grow 12". Retail is of no help either. ID sharks are advertsed as only getting to be about 6-8" which is the "perfect fit" for a 10 gallon tank.
Education is the key ingredient. That's why so many people on this board recommend getting a good fish book and reading about the speices =before= you purchase it. That way you can adjust your tank environment accordingly. Unfortunately retail works against us. They are counting on the "impulse" sale, that the puppy-dog eyes of the 4" oscar will make someone buy a ten gallon tank (estimate sale $150) that day.
So it all is basically up to the individual. I have worked in the retail aquatic world, I know exactly what happens to those "oh I don't want them anymore" fish. It is the same thing that happens to the "oh I don't want them anymore" puppies and kittens. Five days and then euthanised, if they haven't been fed to something upon arrival. We cringe when we think of animals shelters euthanising over a hundred dogs and cats a week, and yet people don't flinch when they think about the number of fish that die in the toilet because their owners lost interest. All the fish I have, save a few, were returns from my pet shop days. I was lucky enough to need to stock my home aquariums, and free fish are always a boon.
Live foods are a different matter entirely. Unless you are vegan for animal welfare reasons, feeding a predatory fish goldfish is no different from slaughtering a cow to make a happy meal.
It would be a bad idea to feed a clown loach to an oscar. Clown loaches have barbs on their fins that could injure an oscar.
~~Colesea