The frag system uses a smaller version of the Model F sump, and has an ATO reservoir to replenish for evaporation on demand.
As you study the picture above, note how the two 1" drains from the tank above drain into both ends of the sump at once. One drain feeds to the far left of the sump, draining everything into the skimmer section. The other drain feeds into the right end of the sump, but a ball valve limits that flow and forces some of the excess water to travel horizontally to the left end, feeding into a Tee fitting. By running the plumbing this way, 75% of the water drains into the skimmer section and 25% to the refugium zone. Another way to think of it is that 100% of one drain goes to the skimmer zone, and 50% of the second line joins that first drain. The other 50% of the second drain runs into the refugium zone. It makes sense if you think about that for a second.
While this image shows a Reef Octopus skimmer in the sump, that has since been replaced with a Nyos Quantum 120 skimmer.
The refugium continues to not be used to its full potential. It needs a light for that section, and some macro algae added to that compartment. It's on my To Do list.
The ATO reservoir has a float valve in it. The 150gpd RO/DI system's tubing runs through the walls into the fishroom, and one line feeds to this reservoir. When the float rises, it stops any additional water from being added and triggers the RO/DI to shut off. At that point, I physically close a ball valve, disabling the RO/DI system for a few days until more water is needed. The ATO reservoir holds about five days' worth of top off water. Using the SmartATO Micro device, water is added when the optical sensor registers a need, and its tiny pump pushes water into the sump. The entire setup is hands-free, other than turning on the RO/DI system about once a week to replenish the vessel with pure water.