Hmm, I dunno how long one would last in a 40 gallon they grow quickly, mostly depends on how much you feed and you'll be tempted to feed that cute little fish as much as it can eat. And believe me they can eat A LOT of foodI had 40B saltwater already up and running, fully cycled (This 40B is home of a Peacock Mantis Shrimp) but I don't want to setup another tank bigger than 40B to grow out the grouper.