When i build or design systems I always follow a few basic rules as follows, and run through a check list on each build to make sure nothing has been missed...this applies right the way through, from desighn to actual running of the completed system.
Allways accept that at some point, anything and everything will break down.
Allways accept, that it will break down usually at the worst possible time..when your away, when the shop is closed, near the end of the month when you have no money left..
1. When the system shuts down (power cut etc) does it go into a stanby state with all equipment still primed and ready to go again as soon as it comes back online..(nitrate reducers are a bit of an oddball here...If they are off too long, then they should be rigged so they have to be manually restarted after inspection)
2. Spread the load, Half the risk.....On crucial circuits like the return pump, try spreading the load across two smaller pumps rather than one big one if at all possible...If one jams up and wont come back on, at least you have 50% of your circulation back after a power cut, not 0%..if you cant use 2 pumps...then choose a pump that has a known capacity to handle solid particulates without jamming up (switch from impellor to volute)
3. Keep crucial circuits (return pumps) on a seperate power feed to any computor controlers...a pump thats powerd strait from a power point will come back on at the end of a power cut...Your controler may not do..(too many links in the chain increases the chances of failyer)
4. Dont tie all your circulation into one controler if possible....If you have 4 circulation devices, split them across two seperate plug bars or sockets if possible. that way, if one trips off, you dont loose all motion within the tank.
5. Never try and do top-up and dosing in one go....It will fail sooner or later...Top up is variable, Dosing should be controled...seperate routes for both....In cases of kalkwasser addition. then simply work out your average minimal daily top up (this gives you your maximum allowable amount), Set your kalkwasser to dose this/or less in a controled manner (peri pump and timer fed from a 'limited' water supply) and let your top up take care of the exess required.
6. Top up holding facilities should be limited to a volume that wont unduly stress the system should it lock on and get dumped (stuck float valve etc)
7.Fans and Coolers are great, but what happens if its a mega hot day and your away....Try to find a way of controling your lights...If you can tie it into a temperature controler so it cuts the lights if the tank gets to a level beyond other means of control, it will add in some extra security....
8. Dont rely on heater thermostats to function long term without siezing on...they will do eventually...Running heaters through a secondery controler is a wise move...especially if its a good quality industrial type thats desighned to be reliable with a seperate temperature probe.
9. Keep all probes and float switches in a dark place where algae wont become an issue (upsetting sensors or jamming the float) and put a reasonably fine guard around them that stops intrusion by critters that could make them fail..(snails love riding on float switches)....doubling up switches is a good way to limit total failyer of a circuit.
10. Go through the system with a fine tooth comb and find the weak spots...know your weakest link and factor that in to any maintanance shedules as 'essential'
11. Carry esential spares at all times.
12. Rigging up at least one circulation pump, one air pump and a low wattage heater to a UPS or backup generator can save a tank from doom in the event of a prolonged powercut...
13. Always carry a bucket of salt on hand....water changes are the number 1 way to save a tank thats had a hard time...
14. Make friends....Other reefkeepers make life for you and your tank a whole lot better if the worst happens,...
Theres loads more, but these are basically my main areas that i try to make people aware of and factor in when building a system...Obviously space is an issue for some so its not always possible to meet all criteria and even then you 'can' get caught out....but knowing where its likely to happen is half the battle even if you dont know when.
Regards