First off - any active shark is going to require a tank/pond that is larger than 2,000 gallons to keep them for life. It all depends on which species you ultimately choose - a non ORV(obligated ram ventilator), can be kept in a smaller space than an ORV can. ORVs require extra room in order to move around and breath - these are the sharks which must contantly be in motion - or they can die.
Leopards are cool water sharks, but are a non-ORV species. They aren't legal to keep below 3' in length, and can reach 6 ft in length. They need a tank/pond that at least 11,000-14,000 gallons (A footprint of 350-450sq.ft. @ a depth of 4') to be kept for life.
Banded Hound Sharks - are basically the Leopard's asian cousin. They have similar requirements to leopards. However they only grow to about 5' in length, and can be imported at a smaller size. They need a tank/pond that is at least 7,600-9.500 gallons (A footprint of 250-320 sq.ft.@ a depth of 4') to keep one for life.
Atlantic Sharpnose are one of the smallest requiem sharks available in the United States. They can reach up to 4' in length - usually average about 39-42" as adults. They are an ORV species, as well as a schooling species. So they need extra space and to be kept in schools of at least 3-5 sharpnose. They require a tank/pond that is at least 15,000 gallons (footprint of 500 sq.ft. @ a depth of 4') for life.
Bonnetheads are similar to the Sharpnose in their requirements - but they can grow to be about 5' in length, and don't require 2-4 other sharks to be kept with them.
Blacktip Reefs are another ORV species. They can grow to 6' in length, and usually average at least 5 ft. As such they require a tank/pond that is at least 25,000 gallons (a footprint of 700 sq.ft. @ a depth of 5') for life.
Whitetip Reefs are not an ORV species. They can grow to 6.5 ft, but usually average about 5.25 ft as adults. They also require a tank/pond that is at least 20,000 gallons (footprint of 530 sq.ft. @ a depth of 5') for life.
Nurse sharks can grow to exceed 10' in length. Although they are benthic(bottom dwelling species) they will eventually require at tank/pond that is at least the size of what a Blacktip Reef requires.
Spooted Wobbegongs are nearly as bad - requiring a tank/pond that is at about 15,000 gallons.
Then there are the smoothhounds - these sharks are smaller relatives of the Leopard & Banded Hound shark. And like those sharks - smoothhounds are not ORVs, but they usually need water temps below 70F. Depending on the exact species - they range from 3-5' in length, most average 3-4'. And so they require a tank/pond that is at least 2,500 - 4,500 gallons (footprint 113-200 sq.ft. @ a depth of 3 ft).
