How Does a Sponge Filter Work? Air, Sponge and Bacteria

Aquarium Co-Op, an aquarium retailer that sells its own sponge filters, calls them “one of the most popular filters used in fish stores, fish rooms, and breeding tanks because they’re so reliable and easy to use.”

What happens inside a sponge filter

hygger, an aquarium equipment maker, lists these stages for its biochemical sponge filter, a model with a ceramic media chamber:

• “The air pump pushes air through the air stone.”
• “Rising bubbles create suction through the sponge.”
• “Sponge filters debris (mechanical filtration).”
• “Water passes through ceramic media (biological filtration).”
• “Clean, oxygen-rich water rises back into the tank.”

Aquarium Co-Op describes the same lift for its own filter, which uses an air collar instead of a stone: “The air pump pushes air through the airline tubing into the hollow cavity inside the sponge filter. Bubbles rise from the inside of the sponge, thus drawing water through the sponge walls.”

The parts and what each one does

Part	What it does	Source
Air pump	Sits outside the tank and pushes the air	Aquarium Co-Op
Airline tubing	Carries air from pump to filter	Aquarium Co-Op
Check valve (when needed differs by source, below)	Stops water flowing back toward the pump	Aqueon, Aquarium Co-Op
Air stone or air collar	Releases the air as bubbles	hygger, Aquarium Co-Op
Lift tube	Bubbles rise in it and carry water up	Aquariumscience.org
Sponge on a strainer (some models)	Traps debris; bacteria grow on it	Aquarium Co-Op
Ceramic media (some models)	Holds bacteria too	hygger
Weighted base (some models)	Keeps the sponge from floating	Aquariumscience.org, of Co-Op’s filters

Two jobs in one sponge: mechanical and biological

Co-Op: “This water suction process mechanically collects debris from the aquarium and gives beneficial bacteria place to grow.”

The sources weigh the two jobs differently. Aquariumscience.org, a single-author hobby reference site, says: “Sponge filters typically have too low of a flow rate to be good mechanical filters but many use them this way.” On the biological side it says: “Simple sponge filters do a surprisingly good job of biofiltration (breakdown of ammonia to nitrate) with a low consumption of electric power IF AND ONLY IF they are decent sized.” Our guide to cycling a fish tank explains how that bacteria colony gets established.

The foam. Co-Op says its sponge “is made with a coarse foam of 20 ppi medium porosity to easily collect particulate from the water without clogging up too quickly.” Aquariumscience.org: “When buying a sponge filter make sure the foam is coarse, preferably 20 ppi (pores per inch) foam or 30 ppi foam.” Of foam that appears to be 40 ppi, it says: “This foam will quickly clog.”

Bubbles, air and flow rate

• Bubble size. Aquariumscience.org says “Most sponge filters use a simple hole to produce very large bubbles that rise in the air lift tube.” and “The many small bubbles of an air stone lift much more water than the large bubbles of the hole.”
• Stone or collar. Co-Op, which sells a collar, says: “Some hobbyists like to add an air stone to the inside of their sponge filter, but it can sometimes slip off over time or allow bubbles to come out of the side of the sponge.” It says its collar “creates a steady stream of tiny bubbles (instead of large, intermittent bubbles) that produces constant lift in the sponge filter”.

Flow rate. Our roundup notes that “AQQA and hygger publish no flow figure for the AQ046 or HG120”. Aqueon, a filter maker, gives a rule for choosing a filter: “Ideally, the flow rate of your filter should be between four and six times the size of your tank.” The same page also says “at least four times the volume of the tank per hour”. Without a published figure, that rule can’t be checked against an air-driven sponge filter (our reading).

Gentle flow. Co-Op: “Because of the constant bubbling, it provides good water circulation and surface agitation, while being gentle enough to avoid sucking up fish fry, shrimp, and other slow-moving creatures.” Our guide to reducing filter current lists what to fit on a sponge filter to turn it down, and our guide to increasing oxygen in a fish tank covers surface movement and air pumps.

Where the air pump goes: check valve and drip loop

The sources word the check valve rule differently:

• Aqueon: “A check valve should always be installed to prevent backflow when the air pump is unplugged or in the event of a power outage.”
• Tetra, an air pump maker, in its 2002 manual for Whisper air pumps: “To prevent possible electrical shock resulting from water back-siphoning, pump should be used with a check-valve or positioned higher than the water level.” Its above-tank setup puts the pump “on a shelf at least 12” higher than the water level of the aquarium, but not directly above the aquarium” and adds “(Note: the shelf should have a raised edge to prevent the pump from sliding off.)” Its next step: “Use a check-valve between the pump and the aquarium.”
• Aquarium Co-Op: “If the air pump is located below the top of the aquarium, you need to add a check valve to prevent water from flowing into the airline tubing whenever the air pump is turned off or the power is out.”
• Fluval, a filter and air pump maker, in its Q.5/Q1/Q2 air pump manual (a copy hosted by a Czech retailer), gives a route for a low pump: “Place the air pump on a surface lower than the water level of the aquarium. To avoid damage from accidentally spilled water, do not place the air pump directly below the aquarium.” It adds: “It is highly recommended to always use a Check Valve (sold separately) with the airline between the air pump and the aquarium”.

Tetra also warns: “To avoid possible electrical shock, do not immerse pump in water. If pump falls into the water, or otherwise becomes wet, do not reach for it until you have first unplugged it.”

For the cord, Tetra says: “To avoid getting the appliance plug or receptacle wet, position the aquarium to the side of a wall-mounted outlet. Arrange a “drip loop” for each cord connecting an aquarium appliance to an outlet.” It defines the loop: “The “drip loop” is the part of the cord below the level of the outlet that prevents water from traveling along the cord and coming in contact with the outlet. (If using an extension cord, the “drip loop” is located below the level of the connector.)” And: “If the plug or receptacle gets wet, don’t unplug the cord. Disconnect the fuse or circuit breaker that supplies power to the appliance. Then unplug the appliance and examine the outlet for the presence of water.”

Air-driven or built-in pump?

	Air-driven	Built-in pump
What moves the water	Bubbles from an outside air pump	A small water pump in the filter
Extra gear	Air pump, airline; check valve rule differs (above)	A power supply
Published flow	None for the air-driven picks in our roundup	48-80 GPH on the four in our roundup

Our sponge filter roundup sums up hygger’s HG176 as “A single-sponge filter with a USB water pump, so no air pump; hygger gives two different tank limits.” and quotes its manual: “THE PACKAGE DOES NOT INCLUDE A DC 5V 1A ADAPTER.”

A third route adds a powerhead (a small water pump) to an ordinary sponge filter; Co-Op sells one that fits its uplift tube. Aquariumscience.org: “It takes the flow rate up.” It cautions: “Note that sometimes hobbyists put a large powerhead on a small 40 ppi sponge filter. The sponge will rather rapidly collapse in this situation.” Fitting one puts your hands in the water: follow the hands-in steps below.

One pick of each drive from our sponge filter roundup: the air-driven hygger HG120, M, which hygger suggests “for 20-55 Gal medium-sized aquariums under 19.7″ depth with an air pump ≥ 2W”, and the USB-pump hygger HG176, S, which the roundup lists for a “tank under 10 gal (size label) or under 20 gal (manual)”.

Rated tank size and uses

Makers rate sponge filters by tank volume, and hygger also by depth and air pump size. hygger’s Small “works pretty well for filtration for 5-20 Gal small aquariums under 14″ depth with an air pump ≥ 1.5W”, and its Large is “suggested for 55-125 Gal aquariums under 26.4″ depth with an air pump ≥ 5W.” Size names don’t match across brands: AQQA, an aquarium equipment maker, says “The small filter is suitable for 5-20 gallon tanks, while the medium filter is suitable for 20-110 gallon tanks.” Compare the gallon figures, not the names (our reading). Aquariumscience.org adds: “Obviously the very small sponge filters are only suitable for small betta tanks or nano tanks.”

hygger lists these uses for its sponge filter: shrimp tanks, betta tanks, breeding and fry grow-out tanks, nano aquariums, quarantine or hospital tanks, and large tanks as auxiliary filtration. Our betta tank roundup compares betta kits on their makers’ specs.

Cleaning without losing the bacteria

hygger says to “Simply rinse the sponge in tank water—no complicated steps.” Your hands go in the water for this. What the manuals say, in order:

Frequently Asked Questions

Air-driven ones do. Built-in pump models, such as hygger’s USB filter, do not.

The sources differ. Co-Op recommends “cleaning your sponge filter once a month or whenever you see a decrease in bubbles (which is caused by the foam getting clogged up with detritus).” hygger says “Rinse sponge every 2–4 weeks (based on bioload).” Aquariumscience.org gives two: for a filter used only as a mechanical filter, “one cleans the filter once a week or so”; for a biofilter, lightly “only when it becomes clogged, like every two months or more”.

No source here compares the two head to head. Aquarium Co-Op, which sells sponge filters, calls them “very reliable compared to other filter types”, and Aquariumscience.org calls them “probably the least expensive filter”. Our fish tank filter roundup compares sponge, hang-on-back and canister picks by tank size.

Aquariumscience.org: “Sponge filters are ugly and obtrusive in the aquarium.” It also says “air pumps can be VERY noisy”.

Co-Op, which sells a coarse sponge, says “Fine sponges often have problems with floating”. It says to “squeeze out any bubbles from the foam if it’s floating”, and AQQA to “squeeze the sponge repeatedly until it is full of water so that the filter can sink to the bottom of the aquarium.” Your hands go in the water for this, so follow the hands-in steps in the cleaning section above: every appliance in the tank unplugged first, plugged back in only afterward.

Sources (10)

• Aquarium Co-Op (aquarium retailer), Sponge Filters: The Easiest Fish Tank Filter Ever
• Aquariumscience.org (single-author hobby reference site), 8.4. Sponge Filters
• hygger (maker), hygger Aquarium Biochemical Sponge Filter
• hygger (maker), hygger USB Electric Aquarium Sponge Filter
• AQQA (maker), Ultra Quite Aquarium Sponge Filter for 5-20 Gallon Tanks
• Aqueon (maker), Setting Up a Freshwater Aquarium
• Aqueon (maker), How to Choose the Best Filter for You and Your Tank
• Tetra (maker), Whisper air pump manual, 2002
• Fluval (maker), AquaClear 20/30/50/70 power filter manual
• Fluval (maker), Q.5/Q1/Q2 air pump manual (copy hosted by the Czech retailer akva-tera.cz)

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