Main tank > canister > chiller > main tank?

[Brock Fluharty]

Hello all. I am in the process of setting up an H. breviceps tank. It is a 20 high, and I plan to have a canister filter rated much higher than that, and a chiller hooked onto it. My question is, if I am just hooking the canister to the chiller, do I need an overflow? Can I just run a length of tubing from the tank, gravity fed to the canister, to the chiller, and then have it pumped back into the main tank, or is an overflow necessary?

Thanks in advance (although my last question got no replies),

Brock