this comment might come a bit late...but .. i had a shark pond before and it contained two black tipped sharks and several bat rays... had loads of fun...and since it was soo large most of the filteration was done naturally... via live rocks and a diatom filter i used weekly. You will need tons of live rock but it will pay off in the end. Had that pond running for over five years... sharks grew too big.... so i had to donate em..