This is from this article
http://reefkeeping.com/issues/2008-01/newbie/index.php
Prevent a Flood
The next subject worthy of consideration is how to keep the floor dry around your sump. If the sump is set up properly, it will not overflow - ever. What has to be established is the total water volume it can hold, and to do that, follow these simple steps:
Each return line in the display tank should have anti-siphon holes drilled into it. I recommend two ?" holes per return, drilled ?" beneath the normal water line when the system is running. The reason for drilling two holes is that invariably, just when you need them to work, a rogue snail is parked over one, munching on algae. Using two holes increases the odds that the siphon will be broken, and reduces the amount of water that back-siphons into the sump. These anti-siphon holes should be checked often to ensure that they are not obstructed with algae or calcification, and cleaned out monthly. In the picture below, you can see water squirting out of the holes as the tank refills. Notice the squares on the left? When the tank is full, the water level is high enough for water to flow through those holes into the overflow, and the anti-siphon holes are completely submerged and silent.
Once the sump is running, unplug the return pump to simulate a power or pump failure. Unplug the skimmer so water drains from its body into the sump as well. After about a minute, the sump's water level should stabilize. If the sump is about to overflow, remove enough water so that it stays full. If the sump still has room, add more saltwater to fill it near the top, at full capacity.
Turn the return pump and skimmer back on, and wait for the display tank to refill and resume draining again. The water level will stabilize at a lower point in the sump within 60 to 90 seconds. Using a Sharpie? or a piece of tape, mark a line on the sump at that water level in the return section. That is the maximum fill line. If the sump's water level is above that line when the power or pump fails, it will overflow. In the picture below, the blue tape is my marker. As long as the anti-siphon holes are functional and the water level is never higher than that line, the sump should never overflow. And as long as the tank's overflow is working properly, the tank won't overflow, either.
