Peter, I'm curious as to how you plumbed that overflow, do you have pics? Asking because my next tank will have an internal yet horizontal overflow like yours..but trying to figure out how to plumb it so that it's very silent, since it will be in my living room. Thanks for any pics or info you can provide.
I just did two 1.5" bulkheads horizontally. Open top tee's attach to these. the 1.5" dual drains go down to a Y and join, since I have pretty low flow through my drains. And then into the sump. Simple design, but I'm not sure it's the quitest. My tank isn't silent, so I don't pay much attention to the drain noise, if there even is any, at such low flow...

