Exactly what d2 said...The drain that you're referring to is your "Open Channel Stand Pipe." This drain serves a dual purpose. First, it's really a secondary drain that handles what your main/primary drain doesn't (which btw.. is very little). It's called an Open Channel because unlike your main drain, it's exposed to air. This air is what keeps this drain from becoming a full siphon. Again, like d2 mentioned, this is why you want your air line just above your tanks normal operating level so that this drain "is not" a full siphon unless your primary drain gets clogged. If this were to happen.....obviously the water will rise in the overflow box, submerge the air line, and turn your Open Channel into a full siphon which basically takes the place of your main drain until the clog is resolved and keeps your wife's new carpet dry! You actual "last line of defense" would be your third drain or EMERGENCY DRAIN. This drain is just a "up turned" elbow or in my case a pipe that's open about 1-2" below the top of your overflow box. Another fail safe that keeps the carpet dry and the wife happy. I did a little video a while back about the bean animal overflow. I'll find and post it below.