Planted Tank Enthusiast
Join Date: Apr 2011
Location: Pasadena, california
If it is custom, go external! You won't regret it and I wish I had, but since your tank will be much larger and wider than mine it will be less crucial.
Beananimal style aka calflo works great! If you are drilling the back or side panel of the tank, just be sure that the box itself is deep enough to accommodate your 90 and a standard screen attached. Mine isn't deep enough to use the screens, so to keep the occasional fish who makes his way back there out of my sump I have to use window screen zip tied over the siphon pipes. It's not a big deal really, it just traps smaller debris than I would care to and requires much more frequent cleaning than the standard pvc screens that come with overflow kits would.
The wider your overflow, the thinner the sheet of water moving over the weir will be. This means the wider the better, because the o2 exchange is based on the surface area of the moving water, and the wider it is the clearer the surface will be. If you can, go coast to coast, or maybe almost the whole way with the returns being in the two corners. You sort of have tobalance the overflow design with your aea esthetic goals. If you don't mind a black background, it is very easy to do coast to coast and hide the overflow plumbing. If you want a more open tank feeling then just put it in the corner of the tank. In your size tank it won't be a huge eye sore.
One idea I had that I eventually want to try would be to set up an external overflow box that goes all the way level to the bottom of the tank, then drill the bottom of the box. This way you could more easily hide the plumbing while keeping a transparent background, which I prefer. None of my tanks are really wide enough to try it, but the easiest way to accomplish this is to give up a bit of front to back depth by placing a pane of glass between the back wall of the aquarium and the front glass. Make it narrow and drill the space between. Viola, coast to coast, clear background, hidden plumbing, and silent bean animal system. Only downside I can see to this is that you would have to clean that overflow glass of algae now and again. And make sure you have access to that area so you can wipe out the overflow box and scoop out any fishes. I would experiment on a cheaper tank first though as this is just my fantasy idea and have no idea what it would be like to run it.
I've noticed that with an overflow you need a little more tank turnover to keep solids suspended in the water column so the skimming action can remove it. My canister run tanks will pull solids from the water column more effectively at lower flow rates. It's all a trade off.
Hope that helps! Opinions are worth what you pay for them.