Do you already have an RO/DI unit?
Also, you might consider getting another phosban reactor and actively running carbon in it. I think it's the best way...but that's just my opinion!
Two Little Fishies Phosban Reactor 150 Pro Package
This package comes with two reactors and the Maxi-jet 400 plus plumbing for it. Quite handy to order it all as a package.