Sump Aquarium Filters in Depth

8.6.1. Sump Aquarium Filters in Depth

The people who are serious about the hobby and have large aquariums use a sump filter in a large acrylic box or glass aquarium, typically located below the aquarium in the cabinet. This box typically has three basic sections (often the middle section is partitioned into several sub-sections).

The first section of this filter is normally either filter bags, polyester floss pads or open cell urethane foam pads which mechanically filter the water. Sometimes this section is alongside the other sections and sometimes this section is on top of the other sections. I skip this section altogether in my sumps.

Symphysodon aequifasciatus Albino Red Discus
Symphysodon aequifasciatus Albino Red Discus

Types of Sumps

Then, based on what is in the middle section, there are five basic types of freshwater sump filters:

  • If the water enters a second chamber where 100% of the biomedia is unmoving and submerged this is a submerged static media sump filter. This is the most common type of sump filter.
  • If the water enters a second chamber where most of the biomedia is unmoving and in air this is a trickle sump filter. A trickle sump is very roughly only about roughly 25% of the efficiency of a static submerged bed sump.
  • If the water enters a second chamber where some of the biomedia is in air and some of the media is submerged this is a wet/dry sump filter. This is roughly 50% of the efficiency of a static submerged bed sump.
  • If the water enters a second chamber where the biomedia is moving and submerged this is a fluidized bed sump filter. Millions of dollars of aquaculture research for fish farms has established this is the most efficient type of sump filter, with an efficiency roughly about 60% more than that of a static 30 ppi foam submerged bed sump.
  • Some people use a hydroponic area in their sump with plants or algae and a grow light of some sort. These systems can remove small amounts of nitrates if they have enough light intensity and the proper plants (emergent plants are much preferred to submerged plants).
Mesoheros festae
Mesoheros festae

Typically, sump water then enters a third section where there is a water pump that pumps the water back into the aquarium. Most commercial units do not include a pump. Be careful in selecting a pump for the return out of a sump. There can be some degree of head (3 to 5 foot) which requires a pump more powerful than most cheap aquarium pumps.

Note these sump filters are typically custom-designed and custom-built in hundreds of variations and combinations. Every hobbyist has their own idea as to the “best” sump filter design. Other sumps aren’t located under the aquarium but rather over the aquarium. Still, other sump set-ups are in another room or a garage. Some even put sumps in the basement of a home with aquariums overhead. These basement designs require careful pump selection due to the high head,

The best and cheapest option is to buy a 40-gallon breeder tank from your LFS. Then buy off eBay a Fiji Cube Sump Kit Refugium Baffle Kit – 40 Gallon Breeder $119.99 + $9.99 Shipping. Anyone, even the DIY-challenged, can follow the directions on this easy kit.

Botia almorhae - YoYo Loach
Botia almorhae – YoYo Loach

Sump Biomedia

A very important consideration in building a sump is the biomedia one is going to use in the sump. Filter biomedia vary by the amount of effective area they give for beneficial bacteria to grow on. The square feet of surface area per cubic foot of biomedia (ft2/ft3) actually available for colonization by beneficial bacteria is very roughly:

surface area
surface area
inches to
get 5ft²
Cubic inches to get 100ft²
Fluidized K1 media
(60% loading in
30 PPI foam in
canister or sump
30 PPI foam
powerhead operated
30 PPI foam air
operated sponge
Plastic pot scrubbers28031620
Static K1 media26033660
20 PPI foam in
canister or sump
20 PPI foam
powerhead operated
20 PPI foam air
operated sponge
Powerhead operated
undergravel filter
Aquarium gravel in
canister or sump
Air operated
undergravel filter
Blue Matala pads120721440
1/8 inch garden
pumice or perlite
Bio balls100861720
1/2 inch lava rock601442880
Seachem Matrix601442880
Biohome Ultimate402164320
Ceramic rings402164320
Ceramic balls302885760
Expanded clay
Cubic inches to get 5ft² = (5/E.A.)x1,728 100ft²=(100/E.A.x1,728)
filter biomedia efficiency

The 5 square feet numbers will give one ammonia oxidation for one pound of fish. The 100 square feet numbers will give one crystal clear water with one pound of fish. These numbers were roughly substantiated by actual experiments done on media under aquarium conditions. Note these numbers are not exact. There are a whole host of variables that can affect these numbers (temperature, pH, food protein level, aeration, biodiversity, etc.). So don’t obsess over them.

Geophagus pellegrini
Geophagus pellegrini

Calculating How Many Fish a Sump can Handle

Now a simple illustration is in order with using mathematically calculated surface area to size a filter and select a filter media. Elsewhere we establish that fish are very healthy and well taken care of by a biofilter media which has very roughly 100 square foot of biomedia surface area per pound of fish. And, since there are 454 grams to one pound, very roughly 454/82 = 6 five-inch fish are in a pound of fish.

Metabolic Weight versus Total Length

A typical 20-gallon sump might have 14 gallons of submerged biomedia in it. The calculations become:

  • X =  # 5 inch fish at 82 grams each
  • Y = Cubic inches of media per 100 square feet
  • 454 grams per pound per 100 square feet
  • 231 cubic inches per gallon
  • 14 x 231 = 3,234 cubic inches available
  • X = (3,234 in3 x 454 gms)/(Y in3 x 82 gms) = 17,905/Y
Heros efasciatus sp Red Shoulder
Heros efasciatus sp Red Shoulder

Let’s put that data into a table of how many five inch fish (82 grams metabolic weight) a 20 gallon sump with 16 gallons being media can support at a level of giving crystal clear water:

  • Fluidized K1             56
  • Foam                         36
  • Pot Scrubbers          29
  • Static K1 Media        27
  • Matrix, Lava Rock      6
  • BioHome, Rings         4

This is simply very incredible data.

Protomelas taeniolatius OB Red Empress
Protomelas taeniolatius OB Red Empress

If one has a large tank and wants large fish what do the numbers say? Let’s go to a 40-gallon sump with 34 gallons of the sump being media. What about ten-inch fish (328 grams metabolic weight)?

The calculations become:

  • X =  #10-inch fish at 328 grams each
  • Y = Cubic inches of media per 100 square feet
  • 454 grams per pound per 100 square feet
  • 231 cubic inches per gallon
  • 34 x 231 = 7,854 cubic inches available
  • X = (7,854 in3 x 454 gms)/(Y in3 x 328 gms) = 10,871/Y

The number of ten-inch fish that can be well supported (crystal clear water) by the media in such a 40-gallon sump becomes:

  • Fluidized K1            34
  • Foam                        22
  • Pot Scrubbers         18
  • Static K1 Media       16
  • Matrix, Lava Rock     4
  • Biohome, Rings        2

Again, pretty mind-boggling data.

Champsochromis spilorhynchus
Champsochromis spilorhynchus

Sump Safety Features

There are five specific safety features needed in any sump:

  • If one has mechanical filtration the sump needs to be designed to prevent overflows it the mechanical media plugs up. To prevent this just make sure the partition for the mechanical filtration is a minimum of half an inch lower than the rim of the sump.
  • If one does not have a drilled tank then one must use a siphon based overflow design. There needs to be two siphon to external weir overflows with clear PVC fittings to prevent failure.
  • With a weir drilled design one needs to be careful the weir doesn’t plug and overflow the main tank. Just make sure the weir is large and open on top.
  • One needs the proper design to prevent a sump from going dry and burning out a pump. This fix consists of simply making sure the pump is in a chamber whose water level is shared with at least 80% of the sump.
  • One needs a proper design to prevent the sump from overflowing during a power outage. This fix involves check valves coupled with vacuum break holes.
Placidochromis phenochilus Tanzania
Placidochromis phenochilus Tanzania

Sumps are prone to flooding if not set up correctly. There are two distinct types of flooding involved.

  • One is where the sump pump moves the water in the sump into the aquarium and the overflow or drilled hole in the aquarium is not functioning. Here the aquarium itself overflows.
  • The other is where the aquarium drains into the sump when the pump is off and floods the sump.

The first type of overflow is best prevented by having two overflows on the aquarium, building these siphon overflows with clear plastic and checking these overflows regularly.  One can also have a small pump chamber which is separate from the rest of the sump with its own water level. This can cause pump burn out but it prevent overflowing the aquarium if the aquarium overflows malfunction. Some pumps have automatic shut-offs which prevent them from running dry.

Flame Hybrid Peacock
Flame Hybrid Peacock

The second type of flooding is where the sump overflows. In any sump when the power goes out or a pump fails, the water from the aquarium can siphon down through the pump in a reverse flow. This reverse flow can flood the sump. The possibility of this is easily prevented with two courses of action:

  • Drill a small one-eighth inch hole in the return flow to the aquarium at the water level. Water will flow out this hole during normal operation. If the pump stops the hole allows air to enter the return piping and break the siphon.
  • Add a check valve in the return, preferably oversized. This prevents rapid siphoning from overcoming the air hole. Note that only a check valve without any siphon break holes at the waterline is not a wise idea as invariable a small amount of debris will hold the check valve open just a little bit. A flow to the sump and flooding will still occur.

Examples of Sumps

Here are just a few of the aquarium sump filter designs which will work quite well:

75% static media 25% fluidized bed sump
75% static media 25% fluidized bed sump

There are eight types of submerged media with one fluidized bed

sump with low fill in the fluidized bed
sump with low fill in the fluidized bed

Another sump with eight types of submerged media and one fluidized bed

sump with three fluidized beds
sump with three fluidized beds

Three fluidized beds with 4 types of submerged media

wet/dry sump
wet/dry sump

Trickle filter sump schematic

Trickle Filter
Trickle Filter

Wet dry sump filter for aquariums with a refugium

Over tank sump
Over tank sump

This is an over-the-aquarium planted sump with a small fluidized bed

Note the use of many types of biomedia is totally unnecessary and serves no purpose what-so-ever. But EVERYONE does it. All biomedia do the same thing, biofiltration. They just vary in how efficiently they do it.

Sciaenochromis fryeri
Sciaenochromis fryeri

What NOT to do

The sump below is a fine example of what NOT to do. There is a lot of empty volume, little biomedia, and the biomedia is largely totally ineffective ceramic media.

A Sump with Very Poor Biofiltration
A Sump with Very Poor Biofiltration


Many get confused in sumps over the issue of how to get the water from the aquarium to the sump. If you have a drilled aquarium the issue is easy. If you don’t have a drilled aquarium the issue becomes more difficult. There are many YouTube videos on making overflows. The King of DIY Joey Mullens has some good designs (His book is simply amazing!).

Some of these designs can be found at this link: Overflow Devices

The selection of a pump for a sump is actually a pretty tricky process. For guidance this link will help:

14.6 Pumps

And the plumbing for a sump is very important, including the piping sizing. This is covered in this link:

14.5 Plumbing and Piping

Dimidiochromis strigatus
Dimidiochromis strigatus

Many want to go into sumps in more depth. Here are links to other articles on sumps:

8.6.2. Static Submerged Media Sumps

8.6.3. Trickle Filters

8.6.4. Wet-dry Sumps

8.6.5 Fluidized Bed Sumps

8.6.6. Do-it-yourself Sump

For those who are cowed by the complexity of a fluidized bed there is a design of sump called a “foam sump” that is very simple in design and in operation. The article about these types of sumps is:

8.6.7. Foam Sump

Startpage Aquariumscience

Source: – David Bogert

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