What is a “modular/shadow type overflow?”
I believe he’s referring to an “external style” overflow box, such as those made by synergy reef!
What is the Shadow® Overflow? The Shadow® Overflow is a device that is used to divert water flow from your aquarium into your filtration sump. This overflow can be used on any Freshwater, Marine or Reptilian habitats. There are no limitations on where you can use this overflow system. Synergy...
synergyreef.com
I have one of these on my 180g, and would buy from another manufacturer if I did it again… the whole thing’s thin, injection molded plastic; mine cracked in several spots as soon as I plumbed it with rigid PVC!