IMO, I do not see the need to run another tank with DSB inside. Reason is that with your good deal of rocks, I do not foresee you having to worry about overload of nutrients. A simple 10% water change should do the trick nicely (:
anyway I am impressed and truly thrilled that you are using a seio M820 as a return. Truly ingenius and I believe it will work out real well kudos to your creativity!
And scrap iceprobe if you are thinking of setting up a 2ft tank. Water volume is too big to do an substantial temperature change (:
Just my 2 cents worth