Both the whitetip reef shark and the blacktip reef shark tend to average around 5ft, you rarely stumble across a 6ft whitetip reef shark. Alot of people would recommend a 30ft diameter tank for adult blacktip reef sharks. The whitetip reef shark however does not need to swim constantly, so a 20ft diameter tank should be fine. Remember these are sharks that you want to keep, try keeping the tank large and remove the corners.