Overflow

To prevent fish, inverts, etc from taking the long ride down the pipe.

Although, Bruce had an anemone that used to like doing that...

It also reduces the amount of water that overflows down the pipe in case of a power outage.

An overflow box allows you to "surface skim" your tank.  Since proteins concentrate at the water surface, it's most efficient to preferentially "skim" this water into the overflow for filtration.

dnellans wrote: that pipe cracks or the seal to the bulkhead cracks and you loose the whole tanks worth of water.  you build an overflow and the bulkhead/pipe crack and you only loose the overflows worth of water.  kind of like double the insurance and well worth it if you ask me.

That's another reason why drilling the top back of the tank is better IMO. Silicon the skimmer box in that area and there is little worry of it getting bumped and cracking the tank or bulkhead. No leverage either, like on a bottom hole mounted durso that has to reach almost to the top of the tank.

Feel free to come see my frag tank with open durso sticking up from the bottom. There really is nothing that will bump it, except my hand. It's only 10" long. A frag tank doesn't usually have a lot of large rock to worry about. If the surface gets a bit of rare scum collecting on it, I simply remove the cap from the modified durso and within minutes the surface is completely skimmed.

The tank space freed up by not having the bulky, full height overflow is the other selling point for me. I always need more fraggin' space.

Mark,

I will probably have to come by and see your setup sometime.

Thanks,

Daren