Questions- horizontal overflows better?

[Chihuahua6]

This question is directed to those with long horizontal overflows across the back of the tank.
Do you see the benefit first hand? I know the theory is that it skims a thinner layer across the surface where the most protein concentration is. Does doing it this way just decrease the velocity of the water being skimmed? Would having two smaller overflow boxes pull the water across faster therefor pulling as much of the surface area as the horizontal overflow would? It comes down to whether the layer pulled into the skimmer is truly thinner and more concentrated with the horizontal overflow or not. A tank builder mentioned to me that they tested the horizontal overflow with dye and felt the difference was mainly in the velocity at which the water was pulled into the overflow and did not see much benefit in doing it this way.
I am going to order my tank this week and these are some of the details I need to figure out. Thanks.

 

[artful-dodger]

The volume of water being skimmed is determined by the output of the return pump. The "thickness" of the water being skimmed is determined by the total horizontal length of the overflow(s). If the overflow is "notched" then the length in this case would be the width of each opening multiplied by the number of openings.

There are other considerations for the horizontal overflow, however. They waste less space in the tank but they can cast a shadow if you are using flourescent bulbs across the back of the tank. When designed in the "Calfo style" (see the All Things Salty forum) they can give you an extra lip across the front where you can mount some spreading corals. they are also, in my experience easier to clean since they are much shallower. Finally, they are easier to retrofit into an existing tank since you don't have to get clear to the bottom to seal the seam.

 

[Chihuahua6]

Thanks for the input. I was considering an external horizontal overflow so that it would not take away any tank space or impede lighting.

 

[artful-dodger]

Very nice approach although I've never done it. (I'm okay with drilling a tank and even cutting straight pieces of plate glass, but cutting a piece out of an assembled aquarium scares me.) Having a custom made tank with an external overflow seems like a great idea.

 

[Chihuahua6]

Thanks again for your input. Do you think you can advise me on the placement of returns and general plumbing? I want to do the overhead return manifold as Anthony Calfo has spoken about on his forum. I think it would be wise to also have some holes drilled in the back of the tank. I just don't know where or how many. I'm not good with the technical aspect of the hobby.