Top 5..or 10... MUST haves for a reef tank

[ReefOcean]

Kind of subjective...

RODI, ATO and Controller are a must have?

Must have for a reef:

Tank, saltwater, liverock, coral, pump, light, heater. Period.

That is how grandaddy did it, and that is the bare bones ones must have. My first 3 years of reefing was done with just that.

You DO NOT need a skimmer
You DO NOT need a wavemaker
You DO NOT need light timers, calcium reactors, Media reactors, additives, UV sterilizers, probes and sumps and doohickies sticking out all over the place

These are options, like the leather and heated seats in Nissans.