What a sump actually does for the bigger tank
A sump is a second, plainer tank hidden in the cabinet, through which all the display water circulates. Moving up to one buys three things a bigger setup eventually needs: extra water volume, which steadies temperature and chemistry; a place to hide the heater, skimmer, reactors, and probes so the display stays clean; and a surface skimming overflow, so the display's surface is always film-free. The refugium is one section of that sump given over to living filtration — macroalgae and a thriving population of tiny crustaceans that consume waste and feed the display. Everything in the build follows from one loop: water falls down from the display, gets processed in stages, and is pumped back up.
The path the water takes, section by section
Draw the loop before you buy anything. Water enters through the overflow — a drilled overflow box is quieter and safer, a hang-on siphon overflow is the retrofit option — and drains down a wide pipe into the sump's first section, usually through filter socks or a roller mat that catch the solids. From there it crosses the equipment section, where the skimmer and heater live in water held at a constant depth by the first baffle, because skimmers only work at a steady level. Next comes the refugium section if the design has one, then a bubble-trap baffle set, and finally the return section, where the pump sits in water whose level falls as the system evaporates — which is why the return section is where an auto top-off sensor belongs.
Baffles are what make it a system rather than a bucket
The glass or acrylic baffles siliconed across the sump are the whole design. Their heights set each section's water level independently: high enough in the equipment section for the skimmer's sweet spot, lower in the return so there is spare capacity above it. Sections are sized by what they must hold, not divided evenly — the refugium wants the largest share after the return safety margin, because algae volume is filtration volume. Most builders dry-fit everything, mark water lines with tape during a test fill, and only then commit the silicone. A sump you can reach into comfortably, with room to lift the skimmer cup out, is a sump that will actually get maintained.
The refugium section, and lighting it backwards
A refugium earns its space by growing something that eats what the display produces. A ball of chaetomorpha macroalgae under a dedicated refugium light absorbs nitrate and phosphate as it grows, and you export that waste physically by harvesting handfuls of algae as it fills the section. Run the refugium light on the opposite schedule to the display — on through the night — and the algae's overnight photosynthesis steadies the system's pH while the display sleeps. The section also becomes a refuge in the literal sense: copepods and amphipods breed there safe from fish, and their young wash over the baffle into the display as live food. Deep sand or rubble in the section gives them housing.
Flood maths and the first fill
Every sump failure story is an arithmetic story, so do the sums before the first fill. When the return pump stops, the display drains down to the overflow's level and the return line siphons back until air breaks it — all of that water lands in the sump, and the sump must hold it with room to spare at its normal running level. Test it deliberately: fill the system, kill the power, and watch the sump rise to its safe line. Set the return nozzle near the surface or drill a small anti-siphon hole so the back-drain is modest. Then run the whole loop for days with a towel budget you do not mind spending, checking every joint, before the cabinet doors close. Once it is live, assign the weekly sump checks — sock changes, skimmer cup, top-off reservoir — in KinsLe Care Teams, where completed tasks are timestamped and the below-tank work stops being invisible.