The main thing in all this is, you not will have just that one fish, and most young beginners will have as many fish tank inhabitants that they can, so in saying that, you really would need to have even a larger tank then a 125, i would go with nothing smaller then a 240, but in dealing with having fish tank inhabitants that would grow 12"-14" if not larger, even a larger tank would be required. If I was to go with a 125 gal tank, I would not cage in such a tank, species that grow so large, it would be like holding a Queen trigger fish in a 125 for it will grow a inch a month after a short while and reach a size to which a 125 would be way too small a tank for it too swim about.
So the idea here for you, that is if your the idea to maintain such larger marine species to first list your marine selection and then discuss what be the best size tank to hold that number of fish and look as well into their normal growth. Also do be aware that groupers will ambush their tank mates, so if your to go with a single grouper species, add it last.