What I would have liked to have known from the outset.
- Everything should be presumed diseased and/or pest ridden until proved otherwise. Quarantine everything 'wet' before it goes in or gets connected to the display system. EVERYTHING: live rock, sand, plants, inverts, corals and of course fish. Look at the staggering number of posts on RC about pests and diseases in display tanks: ich, crabs, Valonia, red bugs, AEFW, etc, etc, etc. ALL are preventable by quarantine, patience and careful observation.
- Properly set up the boring and expensive 'infrastructure' FIRST. Including/especially quarantine. Installing ad hoc tends to make for klunky, finicky or poor performing subsystems.
- The power WILL go out, sometime for days. Frequently in harsh winters. What is the protocol for that eventuality?
- Parts WILL fail. Pumps and heaters are especially problematic. Have spares of all critical parts before putting water in the tank.
- No matter how comprehensively you try to account and budget, how ever much money you think it will cost, it will cost MUCH more.