Used to keep tiny snapper, until, like Wasp, they use to get too big, and aggressive, and I'd release them back into the ocean.
Illegal, yes, will a MAF officer find out and raid your tank, unlikely, and poor form if they did. Was worth the risk, as kept baby trevally, snapper, maomao, red moki, kahawai etc,
Watch your snapper though, put a lovely baby blue maomao in one day, only to watch one of the snapper bit its head straight off.