I need something that stirs the sand up nice and keeps detritus from collecting in my 120g. said:
IME, the easiest way to keep detritus from collecting is to manipulate water flow and use mechanical filters.
The biological way is to use a variety of macrofauna that work together to break down particulate material. It's actually not trivial and requires a broad set of organisms. Dr. Shimek (and others) has good information on this. You can find a little bit of it on the web, but for the full story, you have to purchase articles from him.
Predators, such as sand-sifting gobies, cucumbers, and such, are actually counter-productive to a biological solution to detritus accumulation. Yes, they do eat some of it but their excrement is largely particulate as well, so you're back where you started. They also eat the organisms that are required to further process the waste. When you have miles of sandbed--no problem. When you have 3 square feet--it's a problem.
Even when you do get a healthy, diverse sand bed community going, biological detritus processing is never 100%. You eventually do have to deal with it mechanically.