Do You Need a Sump for a Reef Tank?

What a Sump Actually Is, and How It Works

A sump is a secondary tank, usually placed in the cabinet beneath the
display tank, that the display drains into and returns water from.
The system works as a continuous loop:

1. Display tank water drains by gravity through an overflow into the sump below
2. Water passes through the sump’s filtration sections, mechanical, chemical, biological
3. A return pump pushes the processed water back up to the display through a return line
4. The cycle repeats continuously, the entire system water volume turns over through the sump several times per hour

The sump is where equipment lives: the protein skimmer, heaters, ATO
reservoir connection, dosing lines, refugium, and any reactors. Equipment
in the sump is hidden from the display, doesn’t take up visual space in
the tank, and is accessible for maintenance without disturbing the aquascape.

Total System Volume

One of the most valuable things a sump provides is increased total water
volume. A 75-gallon display tank with a 20-gallon sump is a 95-gallon
system. More water volume means:

• Slower temperature swings, larger thermal mass
• Slower chemistry changes, parameter deviations take longer to develop and are easier to correct
• More dilution of waste products, the system has more buffer before parameters reach problematic levels
• Greater margin for error, feeding mistakes, a missed water change, a heater malfunction all have less immediate impact in a larger water volume

How Water Gets From the Display to the Sump, Overflow Types

The overflow is the mechanism that allows water to drain from the display
into the sump at a controlled rate. There are three main types:

1. Internal Overflow Box (Built-In)

Most tanks designed for sump use have a built-in overflow box, a chamber
built into one corner or the back of the display tank during manufacture.
Water spills over a weir (a notched or smooth wall inside the overflow box)
when the display water level rises above it, then drains through a bulkhead
fitting in the bottom of the overflow box into the sump below.

Built-in overflows are the most reliable and lowest-maintenance option.
Tanks sold specifically for sump use, Red Sea Reefer, Innovative Marine
EXT, Waterbox Marine, use this system. The overflow capacity determines
the maximum return pump flow rate the system can handle.

2. Hang-On-Back (HOB) Overflow Box

A HOB overflow is an external device that clips over the top edge of
a tank not designed for a sump, creating an overflow pathway using
siphon principles. A U-tube or continuous siphon draws water over the
tank rim and into a drain box that feeds the sump below.

HOB overflows work but have a critical reliability weakness: they depend
on maintaining a continuous siphon. If the siphon breaks, from a power
outage, from air entering the U-tube, from pump interruption, the overflow
stops working and the return pump continues filling the display until it
overflows onto the floor. Experienced reefers running HOB overflows add
a secondary siphon break failsafe and a float valve in the sump as insurance.

Beginner recommendation: If you’re adding a sump to a
tank not originally designed for it, a HOB overflow is the only practical
option without drilling, but understand the siphon break risk and plan
for it before running the system unattended.

3. Drilled Bulkhead (Retrofit Drilling)

Glass tanks can be professionally drilled to add a bulkhead fitting that
allows a reliable gravity drain without a siphon. More reliable than HOB
overflow boxes but requires a professional drill (not DIY for most people)
and isn’t possible with tempered glass panels (which most tank bottoms are).
Only suitable for drilling in non-tempered panels, typically the back glass.

Inside the Sump, What Goes in Each Section

Most sumps are divided into chambers by baffles, acrylic or glass dividers
that route water through sections in a specific order. A typical three-chamber
sump layout:

Not all sumps have three chambers, basic sumps may have just a skimmer
section and a return section. The exact layout depends on the sump design.
Purpose-built reef sumps (Trigger Systems, Eshopps, Innovative Marine) are
designed for reef use and come with appropriate baffling. A repurposed
aquarium used as a DIY sump can work but requires careful baffle placement
to prevent microbubbles from the skimmer entering the return section.

The Refugium, The Most Valuable Sump Section

A refugium is a separate protected section, typically in the sump, sometimes
a separate small tank, where macroalgae grows under its own light to export
nutrients from the system. It’s the sump feature that provides the most direct
improvement to water quality and is the main reason experienced reefers
consider a sump essential rather than optional.

How a Refugium Works

Macroalgae (most commonly Chaetomorpha, “chaeto”) grows in the refugium
section under a dedicated grow light. As it grows, it incorporates nitrate
and phosphate from the water into its plant tissue. When a portion of the
chaeto is harvested (removed from the sump) and discarded, those nutrients
leave the system permanently. The harvested chaeto is replaced by new growth, a continuous export cycle.

A well-running chaeto refugium can export enough nitrate and phosphate to
visibly reduce or eliminate nuisance algae in the display, reduce water change
frequency requirements, and support nutrient levels that allow SPS coral growth.

The Reverse Photoperiod

The refugium light is run on the opposite schedule from the display light, when the display is dark, the refugium is illuminated, and vice versa.
Photosynthesis consumes CO₂ and produces oxygen. Running the refugium on
reverse photoperiod means photosynthesis is always occurring somewhere in
the system, the CO₂ and pH swing that occurs when the display lights go
off at night is buffered by the refugium’s active photosynthesis.
The result is a more stable pH across the full 24-hour cycle.

Copepod Cultivation

The refugium also provides a protected environment for copepods and amphipods, the tiny crustaceans that form the base of the reef food web and are the
primary food source for mandarin fish, dragonets, and many wrasse species.
In the display, copepods are constantly consumed by fish. In the refugium,
they’re protected from predation and maintain a population that continuously
seeds the display. A tank without a refugium has far fewer pods than one
with an active fuge.

Sump System vs. All-in-One Tank, The Real Comparison

The practical choice for most beginners isn’t “should I have a sump?”, it’s “should I buy an AIO tank or a display tank with a separate sump?”
These are different system philosophies, not just different equipment choices.

When You Don’t Need a Sump

• You’re building your first reef tank in the 20–40 gallon range
• You want the simplest possible setup to focus on learning the biology
• You’re using an AIO tank, it’s already designed to work without one
• You don’t have cabinet space beneath the tank for a sump
• You’re not planning to keep a refugium or run a chaeto nutrient export system
• Budget is a constraint, eliminating the sump, return pump, and plumbing saves $200–$500

When a Sump Is Worth the Complexity

• Tank is 50+ gallons. At larger volumes, the additional
  water volume and equipment space a sump provides makes a meaningful
  difference to stability. A 75-gallon tank without a sump is a harder
  system to keep stable than the same tank with a 20-gallon sump.
• You want a refugium. If nutrient export via chaeto
  is part of your long-term plan, and it should be for any serious reef, a sump is the most practical way to run one.
• You’re keeping SPS corals. SPS corals require very
  stable parameters and very low nutrients. The additional water volume,
  refugium capacity, and equipment flexibility of a sump-equipped system
  make the chemistry management meaningfully easier.
• You want a clean display. If equipment visible in the
  rear chamber of an AIO bothers you aesthetically, a sump system hides
  everything in the cabinet.
• You’re planning upgrades. A sump gives you room to
  add a calcium reactor, kalkwasser stirrer, carbon/GFO reactor, UV
  sterilizer, and other equipment as the system grows. AIO rear chambers
  don’t.

Sump Sizing, How Big Does It Need to Be?

The general guideline: sump volume should be 20–30% of display
tank volume. A 75-gallon display pairs well with a 15–20 gallon
sump minimum; a 100-gallon display with a 25–30 gallon sump.

Two practical constraints override the percentage guideline:

• Stand cabinet dimensions. The sump must fit inside the
  cabinet with clearance for maintenance access. Measure the interior
  cabinet dimensions before selecting a sump, sump dimensions are listed
  by manufacturers and must leave room to reach in for skimmer servicing
  and equipment access.
• Flood safety volume. When the return pump shuts off
  (power outage, maintenance), all the water in the drain lines between
  the display and sump drains back into the sump. This “drain-back volume”
  must not exceed the sump’s empty headroom, otherwise the sump overflows
  when the pump stops. Calculate drain-back volume (length of drain pipe ×
  pipe diameter × number of drains) and confirm the sump has sufficient
  headroom above operating water level to absorb it.

Running a Reef Tank Without a Sump, What to Prioritize

A sump-free AIO reef tank can be an excellent, stable system. The absence
of a sump means the equipment running in the rear chamber is doing all the
work the sump would otherwise do, so those components matter more, not less.

The most important compensating factor for a sump-free system is
filter floss discipline. In a sump system, a filter
sock catches particles before they dissolve and add to nutrient load.
In an AIO, filter floss in the rear chamber does the same job, but only
if it’s replaced frequently. Floss that’s more than 5–7 days old is
a net nutrient source, releasing more dissolved nitrate and phosphate
than it captures. This single maintenance task has more impact on water
quality in an AIO than any other routine action.

Quick Reference, Should You Use a Sump?