as stated above, it's definitely a gator gar. these fish won't grow a foot/month in captivity, but can reach around 3' (give or take) in the first year given the right conditions.
either way, this species is by far the largest AND most aggressive of the gar species. it will grow fast AND it will take out those tankmates in the next couple months, if not sooner. gators have an immense drive to feed, especially early on in life, so i would first off consider getting a smaller species of gar if possible (maybe trade in for a Florida/spotted). they will get big fast and they will take out tankmates...two issues that do not make them very well suited for most aquarists, and beginner gar-keepers.
i'm also curious as to the size of the current tank, we can give you an estimate of how long the fish can stay in there if you let us know. great fish, just not great for most hobbyists--
--solomon