Reducing Overflow Noise

[Greybeard]

I can't see the outside overflow box, but I'd guess your water level problem starts here.

Water height in the in-tank section of these HOB overflows is usually determined by the height of the outlet on the out-of-tank portion... the tube in red in the crappy drawing I did. Extend this tube to the same height as you want the water level to be in the in-tank compartment, you should be able to quiet it down some.

Note: I DO NOT recommend these devices. None of them. If my choices are (A) No sump, or (B) a HOB Overflow, I'll take option A every single time. I simply do not trust HOB overflows. One too many flooded floors. Never again.

 

[Reef-junky]

Oh great! Is that bad for the pump to be dialed back like that?
What about some good small wave makers to supplement flow?

T off the return pipe and run a valve on the pipe going back to the sump to bleed off speed. For return flow 3x to 5x is the norm. Some people go lower but I'm not a fan of that. I look at the total volume (I count the sump) but a lot of people don't. To me I want to turn over the total system and the water in the sump is part of that. It's not as if the sump is isolated some how. 10x used to be the old standard.