OP please read this and please do not release your pet into the wild.
www.fishtanksandponds.co.uk/ethics/releasetowild.html
"Releasing the fish in to the wild
If you were to do this then the best scenario is that the fish quickly dies without passing on any infections. If it is a tropical fish and it is Summer then it may live a few months before slowly dying as the temperature drops, probably not what you had in mind eh?
If it is a coldwater species then it is entirely different. It is likely the fish would thrive if it could avoid predators and if it were to meet another of its species which isn't to unlikely because fish are sold in huge numbers and unfortunately quite a few are released.
If a foreign species becomes established it will displace some of the native species as has happened with Grey squirrels, Mink, Signal crayfish and so on. All of them have done untold damage to our native fauna through being released in to the wild, ironically in the case of the mink since it was released in to the wild by well meaning animal activist, the mink is responsible for bringing the once common native water vole to near extinction.
Signal crayfish have almost completely displaced the native crayfish because they carry a virus which doesn't affect them but the native crayfish has no defence against.
The grey squirrel has all but displaced the native red squirrel by simply being larger, more aggressive and more adaptable
This probably all seems a little extreme and unlikely just by you releasing your one fish in to the local canal or pond. But that really is all it takes. Some bacterial diseases for example can wipe out 90% of fish stocks in an area within a few days and the host doesn't always show signs of a disease.
Finally it is illegal to release fish in to the wild, even native fish can't be moved from one location to another without approval from DEFRA. The penalty for doing so is a fine up to £2,500.00. "