Obviously what you are saying is 100% correct. I think he a little confused because in his alligator gar thread he was told that his 80 was too small for a florida. Hes gone out and bought a bigger tank and now this one is too big. like oddball says mgm you need to start with a small tank and work up to the 150. Though depending on the dimensions I tend to agree you may struggle size wise in the long term.Don't know what you're mad about. It's correct that an adult gar would need a large tank. However, small gars (and any young fish) have increased survival instincts due to being prey to many more species (their own included) because of their size. Young gars hide out more than adults to avoid becoming snacks for any large fish. They will eventually relax as they grow since increased size inevitably leads to being at risk from fewer and fewer species until they're large enough to be a risk towards most of the species their instincts made them fear as juveniles.
As juveniles in captivity, a smaller tank means less threat of injury from their fight or flight instinct. Young gars will jump at shadows and that jump usually leads to broken jaws or backs from impacting the walls of the tank. A smaller tank would restrict the speed they can achieve during a dart for cover. Less speed in a smaller tank means less impact trauma.
As the gar matures and becomes settled in its environment (inside and outside the tank), it can then be moved to larger housing.