Once you get over 200 gallons, youre in a whole nother world. The under tank sump and filtration is possible, but maintenance and access becomes a larger factor relative to efficency or cost savings.
In no particular order - except the order I thought of them in....
#1 dedicated electric circuits
I have 2x15A. I wish I had 4x20A
#2 backup for power outage
APS Powerverter at a minimum
Gas generator preferrably
At the very minimum, plan to run an air pump on a PC backup APS and bubble air to keep the water oxygenated. It's even better to keep the main circulation and heat on.
#3 humidity control
Either a vent fan on a controller, or an air exchanger. Dehumidifiers work, but add too much heat in the summer.
#4 have a sump room of at least 75 sq ft
I do something new every year and appreciate my 150 sq ft sump room. When I want a new 120 gallon fuge, or 72x24x10 frag tray, or a new monster skimmer, or a dedicated fish food & beer fridge, or a fragging workstation, or new monster skimmer... you get the idea...
#5 Train someone else to take care of it
Teaching my dad how to do general maintenance and what problems to look for has saved the day a few times when I've been out of town.
#6 Keep is simple
The first go round, I was searching for every new techie toy to make the Uber-efficient tank. It works a lot easier if you start with a simple balanced system and add the techie stuff later. It also gives you a basis for comparison...
Fictitous example: my tank ran great on the old euroreef with the caulerpa refugium. Now with the bubbleking and phosban and caulerpa, it's actually harder to maintain because of X, Y, Z.
I still don't use my controller to the complex level I had originally planned. All said I'd be happy with a few independant pH controllers and an ORP monitor.
Using a controller for lights is iffy when you get over 1000W. I prefer to seperate my lighting across circuits, and use outdoor rated mechanical timers - even after trying the cool shutoff and restart features on the more advanced controllers (I've had Aquatronica and Neptune).
#7 Plan to expand
OK, that's kinda the flip side of #6. Keep it simple, but plan that you'll add all kinds of crap. The best way to do this, IMO, is to create a header on the pump and on the drain line back to the sump.
I now have about eight 1/2" outlets and two 1" outlets on my main pump all with ball valves. The 1" are for the main tank. The 1/2" are for other tanks or devices (refugia, frag trays, Ca reactor feed, Skimmer feed, etc) I currently have 5 spare lines, but will soon be down to 3.
I have the same deal with my drain lines back to the sump, but 2 are 1.25" from the tank, and the rest are 1". All with ball valves.
#8 Go slow
They say nothing good happens quickly in a reef tank. Whoever they are, they're usually right. So whenever you do a significant change, try to phase it in. Talk to anyone who dumped a pound of ROWA Phos into a reactor.
This also goes for adding livestock. Just because you have a ton of rock does not mean the bacterial colonies can instantly adjust to 10 big fish + feedings or adding 50 frags at once.
#9 QUARRANTINE
Possibly the most important. Especially on a large tank. I have myrionemia species hydroids because I was impatient and didn't quarrantine a few frags. It started as a few small bits of hydroid runners that I didn't see on the frag plugs - now it's a pain in the arse.
