What are the MUST HAVES for equipment?

[Coralgurl]

What would you absolutely not run your tank without and why? Obviously lights, heaters, skimmers, ato's. I've got a 180 gl dt with an 80 gl sump, I'm looking for info on controllers, reactors (I've got a BRS dual GFO and carbon running), dosers and whatever else you use that improves the quality of your tank and its inhabitants and easier on you.
Thanks