One of the absolute most irresponsible things that can be done, as a hobbyist, is to release them.
There are many species introductions that occur this way, with well meaning people creating potentially huge problems (yes, the snakehead is the perfect recent example). Please do not do this. Do not BUY fish that you can not house long term. that is the easiest solution. Research the fish. But never, ever release them, rock, algae, corals or anything to the wild. It is your responsibility, if you can not keep them, to find a different home, or to kill them.
The only time you might be able to do this, is if you are keeping a local tank, with things you ONLY collected locally (rock, substrate, fish, inverts) and nothing introduced from another region. But this is very rare, and even it has implications.