Whether you are keeping them with tropicals or not it is illegal to re-release them.88GT;1065934; said:
Even if there are absolutely no contaminants in the water you are growing out a fish that may or may not have gotten eaten or died in the wild.
You've taken a potentially weak specimen, kept him well fed, grown him big so that his chance of being eaten or killed is significantly lowered, and released him back into the wild where he'll potentially breed and pass his weak genetics on to future offspring. Now that he's nice and big and has been kept alive by you he's able to take over territory from smaller but genetically superior fish because perhaps if you wouldn't have caught him mother nature would have taken care of him before he got the chance to breed.
The risk is significantly raised if you release him into a different waterway than he came from, but it's still there even if you release him back into the exact spot where he was found.
He could also still contract a foreign parasite or disease whether you keep him with non-native plants or fish, or ANY other fish or plants for that matter. What are you going to feed him? You can't make sure you're getting food from the exact place where he'd be eating it in the wild.
There's just too big of a risk keeping him in an enclosed system where he'll stay in close contact with any parasites in the water raising his risk of becoming infected.
It's just overall a bad idea to release fish that have been kept in an aquarium back into the wild. Ecosystems are fairly complex and (re)introduction of anything into them could cause problems for certain creatures.
The best bet is either not keep one if you can't provide a home permanantly (best choice), euthanize him when you can no longer provide space(provided you can't find somewhere to put him other than local waterways), or eat him. Releasing him back into the wild should be out of the question.
There are plenty of natives that can be kept in that size aquaria for life.
