I found the most important information that I wish I had really been aware of when I started my reef tank is this -
*Know about everything you plan on putting into your tank BEFORE it goes in there. Do your own research and do not listen to the local fish stores about too much. Know all the pros/cons of adding each coral/fish/invert into your tank.
*Wait.. be patient.. do not rush once you're cycled.. let things settle after each new entry into your tank.. you may want to add another fish every day or sometimes more than one at a time.. but its really not a good idea. Trust me
I made these mistakes.
*If you research something and it says 'expert' difficulity to keep something.. just dont get it.. do you think you're gonna be able to keep something where others have failed? Its possible sure.. but no point in trying when you're starting out.
*Test kits.. spend the money on some good testing gear.. this is REALLY important and it took me a long time before I realised this and lost money because of it. Not just the basic tests either.. some of the ones that fish stores will tell you 'oh you dont need those'... (yes you do). This includes a good refractometer.. but I know those are $$ so at least a good hydrometer (salinity testers).
*Have backup plans ready... what if your AC goes out in the house? Have a plan to fix any tempature problems... what if you get a leak in your tank? Have backup RO water prepaired. What if a powerhead breaks.. have one on backup. Etc.. always be ready for something awful to happen
Cause it can.. and will happen.