I work on a 70 gal tennor at work (which has what you are refering to). I took the bio-balls out, they are a Nitrate generator. you have live rock so you really dont need them. You already have a type of sump in place, you could drop some live rock in there if you like. Was there some special resaon you wanted a sump? you could still use what you have (less the bio-balls) and plum underneath and behind your tank into the bulit in sump and not mess your tank up.