[untamed]

I realize that you haven't built the overflow box yet, but your plumbing mock up shows the primary (Herbie) overflow intake as being quite high in the tank. You don't want that. That line should just be a bulkhead right on the bottom of the tank.

Your emergency flow drain should have a vertical riser like that. The surface will be skimmed by the overflow box, and the water level in the overflow box will be between the bottom of the tank and the level of the emergency overflow.

The closer to the top of the overflow box that you run the water level (controlled by your gate valve), the quieter the overflow box because the only noise is the water falling from the tank surface down to the water level in the overflow box.

Optimally, you will want the water in the overflow box to be almost as high as the tank surface, with the emergency drain standpipe, just higher than that.

Finally...consider using the Herbie AGAIN for the water that drains from the refugium into the sump. As is, you'll have silent flow right from the tank to the refug and sump, but the water that drains from the refug to the sump might be noisy. Use the hole you've already made as the emergency, and create another bulkhead at the bottom and run it as a Herbie. That's what I did and it works perfectly.

There are actually quite a few similarities between what you are doing there and what I have already done. You are welcome to drop by and examine and listen to plumbing, if you like. (I don't know how close you are to Coquitlam...)