Connecting frag tank to display tank?

Hey all :) I am contemplating setting up a frag tank. Now I don't really want another tank to take care of so it came to me that I may possibly be able to hook up the frag tank to my display tank somehow. What I am thinking is somehow set it up to where the water runs into both tanks. So they would have same water quality and such, just differnt lighting. I don't know if that's even possible though as my 120 gallon has been up and running for 2 years or so. I am not skilled in the plumbing department either. I more then likely won't end up with a frag tank unless this works out. I do have a 15 gallon lying around that's plumed in the back already, I could use it. So is this idea possible at all? Do I need to explain what I am thinking of a little bit more? Thanks!