My first reef tank

Hey all! I am just starting out with my first reef tank. I have a 55g freshwater, planted, guppy and snail tank that is thriving but wanted to step up my game. I purchased a 120g with stand and sump. Then bought a roughly 20-30g tank for external refugium (cause I think they look cool) and setup all my plumbing for both. I bought a new 200W heater as my LFS suggested 2 small heaters instead of 1 large Incase of failure of the 1 large. I have 2 large powerheads, a used protein skimmer from LFS (don***8217;t know the name). An RO system with a 55 gallon reservoir and 44g mixing bin. I posted in then looking to buy forums for another heater and some live rock if I can find it cheaper than my LFS. Ultimately I***8217;m looking to see what I***8217;m missing in the minds of the the master reefers. What items are essential going forward other than the obvious water, rock, and fish. And what pitfalls should I watch for that you have experienced and would save me some unforeseen headaches. Thank you so much for any input and advice.
The saltwater newb. ***x1f609;