Ideally you step them up as they grow.. 10 gal to 50 gal to 150 gal to 700 gal is a quick off the top of my head good example..
Don't try to jump a small fish into a big tank.. This often does not work well...
When gars are small they need regular available feed which is hard to do in large tanks..Once the fish reaches a size of over 10 inches this lessens somewhat, as by this time, they are well developed and if you do things right on prepared foods. For gators this will work good.. Don't try it with a Lepisosteus as these fish will not do as well at even 10 inches in such a stock tank without more specialized conditions..