Ideal location for filter outlets in my stingray / arowana tank

Hi all,

I'm currently building a large tank (2900gal) and wondering where would be the ideal location(s) to place my filter outlets (will be set in blockwork). A bottom drain would have been ideal but as I have now decided to put a sand bed on the base, I'm going to have to locate them a little higher so as not to suck up all the fine sand.

Any suggestions?