i would take it down to your local fish store and maybe they'l take it off your hands and if they have a huge display tank they can use it there. or you can take it down to a river or lake and release it....
My local aquairum was gonna take mine when it out grew my tank you can always ask them also if they have a big freshwater display... sadly mine died before it got that big =(
Even if the type of fish is local to your area, you shouldn't release it after it's been in a fishtank. You never know what form of mutant, microscopic nasty you would be releasing into the population, and you could (on an outside chance) devastate the local ecosystem with it.
Do not buy juvenile fish that you cannot house at their adult size.