Hello; If you are fishing and catch a fish there should not be a problem with releasing the fish.
I am forced to guess that you are talking about taking a fish home, keeping it is a tank for a while and then for some reason decide that you no longer want it. Is this correct?
If so, then the first answer is that it may be illegal to release such a fish. check your local and/or state laws.
My guess is that another reason is that the potential for spreading disease or parasites is a factor. Should some disease or parasite be in your home tanks it may be introduced into the local waters.