What a Sump Does and Why It Outperforms In-Tank Filtration at Scale
A sump is a secondary tank connected below the display tank by gravity overflow, creating a closed-loop system where water falls from the display into the sump, passes through filtration chambers, and is pumped back via a return line. The fundamental advantage is water volume — a 300-liter display tank with a 100-liter sump effectively behaves as a 400-liter system for dilution of waste products and buffering of pH and temperature swings. Temperature fluctuations that might shift a standalone 300-liter tank by ±1.5°C during a hot day are reduced to ±0.3°C when the full 400-liter system volume must change temperature.
Sumps also centralize equipment that would otherwise clutter the display tank — protein skimmers, heaters, dosing pump lines, carbon reactors, UV sterilizers, and refugium chambers all fit inside the sump cabinet. This reduces visible equipment in the display to the overflow box and return nozzle only. For reef tanks specifically, the sump allows protein skimmer operation at consistent water levels independent of evaporation in the display, since the auto top-off (ATO) system refills the sump rather than the display.
Calculating Overflow Rate and Selecting the Right Overflow System
The overflow system controls how much water moves from the display tank to the sump per hour — this is the system flow rate and must match the return pump flow rate for the system to reach equilibrium. For a reef tank with high-flow corals, target 5–10 total system turnovers per hour: a 300-liter reef tank needs 1500–3000 L/h of overflow capacity. For freshwater planted tanks, 5–7 turnovers (1500–2100 L/h for 300L) is sufficient. Overflow systems include: internal overflow boxes (most common, quietest), external overflow siphon boxes (no tank drilling required, higher siphon-break risk), and drilled bulkhead overflows (most reliable, requires drilling).
Herbie overflow systems — two drilled bulkheads per side, one set as a full-siphon drain and one as a dry emergency drain — are the quietest drain configuration available for drilled tanks, eliminating the characteristic gurgling of single Durso standpipe designs. The full-siphon drain uses a valve to throttle flow to exactly match return pump output, while the emergency drain sits 2–3 cm higher and only activates if the primary drain clogs. Herbie systems operating correctly are nearly silent — a properly tuned system produces only a faint hiss from the drain standpipe.
- ✦Size the sump at 25–40% of display tank volume minimum — a 300-liter display needs a 75–120-liter sump to provide adequate refugium, equipment space, and evaporation buffer.
- ✦Never run a return pump rated above 120% of your overflow system's rated capacity — excess return flow overwhelms the drain and floods the display within minutes.
- ✦Add a gate valve on the return line after the pump to fine-tune flow without needing to swap pumps — this is the single most useful plumbing addition on any sump system.
Three-Chamber Sump Design: Filter Sock, Equipment, and Return Sections
The standard three-chamber sump layout divides the sump tank into: (1) a filter sock or mechanical filter chamber where raw overflow water first enters and suspended solids are captured, (2) a central equipment chamber housing the protein skimmer, heater, dosing lines, and any reactors, and (3) a return chamber where the return pump sits and draws clean, processed water back to the display. The baffles between chambers control water direction — water from the filter sock overflows or flows under baffles into the equipment chamber, then over a final baffle into the return chamber. Baffle heights determine minimum water level in each chamber.
The filter sock chamber should have a roller mat filter or a removable 200-micron filter sock. A 4-inch filter sock on a 1500 L/h system requires rinsing or replacement every 3–7 days in a moderately stocked tank; an automated roller mat (Clarisea SK5000) advances clean filter fleece automatically when flow restriction is detected, reducing manual maintenance to roll replacement every 2–4 weeks. Skipping filter sock maintenance causes particulate matter to reach the equipment chamber, clogging protein skimmer necks and coating heater surfaces with biofilm.
- ✦Paint the sump exterior with black aquarium-safe paint or wrap it in black vinyl — light penetration into the sump causes nuisance algae growth on all equipment surfaces within 3–4 weeks.
- ✦Install a sock or strainer on the return pump intake to prevent sand, broken snails, or small organisms from entering the pump impeller.
- ✦Keep the return chamber water level stable by placing the ATO sensor float here — sump water level changes with evaporation 3–5x faster than display level due to smaller surface area.
Refugium Chamber: Chaeto Macro-Algae and Copepod Cultivation
A refugium is an optional fourth chamber (or a separate small tank plumbed into the sump circuit) dedicated to cultivating macro-algae such as Chaetomorpha linum (Chaeto) or Caulerpa racemosa under dedicated lighting. Chaeto grows rapidly under a 6500K LED rated at 30–50 PAR, removing nitrate and phosphate from the water column through biomass uptake — 100g of Chaeto harvested weekly from a reef sump can export 5–10 ppm of nitrate equivalent from a 200-liter system. Run refugium lighting on a reverse cycle (on when display lights are off) to buffer overnight pH swings caused by plant respiration.
Refugiums also serve as safe breeding and cultivation chambers for copepods (Tisbe, Tigriopus, Apocyclops species), amphipods, and microfauna that form the live food base for mandarin dragonets, seahorses, and finicky reef fish. Seeding the refugium with a 60mL pod culture starter weekly and maintaining Chaeto as substrate allows copepod populations to reach 50,000–200,000 organisms per liter within 6–8 weeks — enough to sustain a single mandarin dragonet in a 200-liter display connected to a 60-liter refugium.
Return Pump Selection: DC Variable vs AC Fixed and Head Pressure Calculation
The return pump must overcome total head pressure — the sum of vertical lift from sump water level to display return nozzle plus friction losses in the return pipe. For every 1 meter of vertical lift, subtract approximately 10% of rated pump flow. A Jebao DCS-3000 rated at 3000 L/h max flow pumping 1.5 meters of vertical head delivers approximately 2100–2400 L/h actual flow at that head pressure. Always calculate required flow at your actual head height and select a pump that meets target flow at that head — not at the zero-head maximum rating on the box.
DC variable-speed return pumps (Jebao DCS, Aqua Medic DC Runner, Royal Exclusiv Red Dragon) offer flow adjustment from 20–100% speed via controller, allowing fine-tuning of system turnover rate after installation without replumbing. AC fixed-speed pumps are cheaper but require a gate valve to reduce flow and waste electricity at reduced settings. For systems above 200 liters, a DC variable pump saves 30–50W of continuous power draw versus a gated-down AC equivalent — approximately 250–440 kWh per year and a meaningful reduction in heat input to the sump water.
- ✦Run a new return pump at full speed for 48 hours before finalizing plumbing — some DC pump controllers require initial calibration runs to map minimum and maximum flow curves accurately.
- ✦Install unions (threaded disconnect fittings) on both sides of the return pump for tool-free removal during maintenance — hard-plumbed pumps require cutting PVC to remove for cleaning.
- ✦Check return pump impeller every 6 months for calcium buildup in marine systems — soak in undiluted white vinegar for 4–6 hours, then rinse thoroughly before reinstalling.