I was thinking in terms of overflows, sensors, and relays. I have a sump, refugium in the lower level (downstairs), so it would be good to shut off the power to the lift-pump in the future should the tank malfunction. I've heard of level sensors in case the overflow clogs.
Also, I'm pretty sure the lime-water topoff killed my fish. It would be good to kill power to that too if something goes wrong in the future. I was able to save most of them, but then they died from something (Ph?) over the next 48 hours. Only two survivors. Oddly, the coral and anemones made it.
I'm just thinking of beefing up the redundancy and then adding a level sensor and relays / computer.
Redundancy is never a bad thing. Whenever you have a vital sensor such as a liquid level or thermometer, a backup will eventually come in handy.
For worry warts like yourself (myself included) a basic controller can help you sleep better at night. It doesn't have to control everything, or even monitor everything, rather just a few things. Putting simple failsafes in place like turning lights off if the temp goes above X, or shutting down the ATO system at a max level or max pH, will make you worry less. For that your easiest solution is an off the shelf ready controller like a Reefkeeper (Digital Aquatics) or Apex (Neptune) combined with a few accessories like float switches and probes. If you want more control, the sky is the limit, it all depends on how much you want to spend, and what you can design and fabricate.
One other product to look at is a simple water detector, it's a small device that has a water sensor, you put the sensor on the floor in the stand, and it'll emit a loud beep like a smoke detector if water makes it's way to the stand. If you're home, it's good, if not, you'll come home to a loud noise and a wet floor.
One of the most important things to note is to keep your system simple, avoid using unions, check valves and external equipment if at all possible. When plumbing, use solvent pieces rather then threaded, PVC is silly cheap, so having to cut it to move the tank won't hurt your wallet. For reactors, skimmers, and everything else, if it can fit in the sump, put it in there, the O-Rings are reliable, but at some point they will fail, and you likely won't be expecting it.
