I'm of the opinion that stability in a reef system is key to success, and that an ATO is the single most important, and effective, piece of equipment you can purchase to increase stability. Ok... a heater is probably first, but we all have one of those, right?
Unfortunately, ATO systems, like all mechanical devices, are prone to failure. I like to see the following features in an ATO system:
1) Limited volume in reservoir. In my 150g system, my ATO reservoir holds 5 gallons of water. If it all dumps in at once... well, it's just not that big a deal.
Hook your ATO up to your RO/DI directly, or to a huge reservoir, and a failure can not only wipe out the tank, it can cause major water damage.
2) Redundant water sensors. I use a Tunze, it has an optical sensor, an electrical float switch in case that fails, and a timer that'll stop the pump if BOTH fail. Any good ATO should have at least two sensors, a primary, and a backup, because the primary WILL FAIL at some point. In my case, I have an additional high level sensor in my sump, tied to my Apex. If it trips, it cuts the power to my ATO, as well as shutting down my skimmer, so that I don't flood the skimmer cup.
Note: Using Kalkwasser in your ATO dramatically increases the importance of both of these points. 5g of RO/DI water? no big deal. 5g of saturated kalkwasser is another thing entirely. I used to do this, and stopped, some time back, in favor of dosing a known quantity of Kalk via a dosing pump. Much more stable results.
