Hey Chris,
Let's play, change the name game.
In the aquarium field, a
weir goes across the entire length or width of the aquarium. It's usually 4" deep and 4" out from the back or side wall but, could be any dimension the designer wants. Bulkheads are installed in the wall below water level and water drained externally to the
sump below. I've seen them internal and external, with internal being the better design. The weir is not a bad design.
Internal and/or external overflows can be various lengths, depths and widths depending on the size and number of drains and return lines you intend on installing. Usually, but not always, they extend from the bottom to the top of the aquarium and are slotted to keep most fish and crawling invertebrates from entering. I've seen the external overflow (glued to the back wall) used a couple of times and felt it would be vulnerable to breaking. It also made building a stand more challenging. The internal design is a better design and the most often chosen option.
The
external overflow syphon box is a hang-on device that is usually chosen by those that cannot or will not drill their existing aquarium for whatever reason. This option is the least reliable due to syphon breaks, flooding and restricted flow. The worst option!
As long as you are designing and having custom made the aquarium of your dreams, go with the internal overflow. It's the most reliable in performance, cheapest to have installed, and least prone to failure.
I hope this cleared things up for you.
Dick
