For me it isn't the loss of light for the corals, it's the shadow it cast on the rear glass. Also the typical C2C box can be seen from the sides of the tank. If I was doing an in-wall tank I would probably use a C2C will a tank long external box, but mine can be seen from both sides and I don't like being able to see into the box, whether internal or external. My internal box is going to be 42" long for my 48" tank and is mostly smooth. I understand why the height of the external box is important, but why does it need to be larger in length and width? Not trying to be difficult, just trying to learn.
You have a decision to make, and it is your decision, not mine. Aesthetics vs function and system health, in terms of an internal box. As far as the box being seen from the ends of the tank, well folks still flock in droves to spend their money on reef ready tanks, where the end overflow exposure outside the tank is the full height of the tank. The C2C is about function and efficiency, not about what you can or can't see from the ends of the tank.
The light loss from a C2C is insignificant, and the back of the tank being a tiny amount darker, makes we want to ask: what are you looking for back there where nothing should be? I run my lights generally around 6" short at both ends of 6 foot tanks, to make the ends a "darker" area for less "light loving" critters, as well as the back being somewhat subdued. Folks want to paint the backs of their tanks black, to make the back darker, hide pipes etc. That reduces the amount of light in the tank more than a slight shadow from an overflow box. Where you put the lights makes a difference as well, all of it makes a difference. It is an aquarium, it is not possible to make it look like a natural ocean, it only makes things hard on you if you try. We don't have the 100,000 gallon luxury of public aquariums where everything is hidden.
The height is pretty much self explanatory. In the conversation 5 years ago, it was make the external tall enough above/below the through holes to give enough vertical adjustment room to get things dialed in. This is called "tuning" the system. The water level on both sides, at the widest part of the bare 3" through holes, (center line) rather than hanging the overflow box off of bulkheads.
Longer in length so you have room in the box, and are not jamming everything together. Length and numbers of holes, make the internal and external act more as one body of water. Width less of an issue. This is becoming a case where folks are just way over thinking things, making things hard on themselves. An 18" external box on a 48" tank rather than a 12" box is a problem because? (numbers pulled at random.)
Folks are not going to notice the OF boxes, or the plumbing if the husbandry of the system is first rate, and the display stuns them into a silent stare. I think that should be the focus, rather than scratching ones head figuring out how small of a foot print can I cram this stuff into. There are advantages and disadvantages to just about everything. It is sorting out what is really important, and what just is not all that important, in terms of system performance.
This thread, and my "job" is to figure out why someones drain system does not work. The list of reasons is very short.
