in australia on the sunshine coast we have an exhibit called underwater world. they keep all kinds of fish but the main tank holds almost 20 1metre + barramundi and a mangrove jack of equal size. full fresh. yes in the wild they do tend to move offshore but they do not require it. its not like it has a switch in its body that will kill it if it doesnt move offshore. and im sorry to sound rude if i do but 20 plus kilos is a big fish my friend. mangrove jacks, even from a reef do not exceed 10 kilos