While there are some species -like Bamboos, Coral Cats, Epaulettes, or Horns that may be kept in a 125 for up to a year (as pups).
There's present no Species of shark that can be kept for it's entire life time in any tank/pond under about 220 gallons (6'L x 2.5'W).
So if you were to get a shark - you will have to upgrade.