Ok a 120gal would not be big enough for the even splitting them up. Also you need to do at the very least a water change every week especially with all those fish. Also with the Gar never release aquarium fish into the wild you can contaminate the wild species with diseases that they would not be able to fight off. I would take the Gar back from were you got it unless you fished for it and tossed it in your tank then Ide wait til hes of size and then eat it. Ummm Ummm Good. But yeah never ever release aquarium fish with the wild population even if there native there. Also if your caught doing this you get hit with a big fine.