How to Use Box/Corner Filters

Its Purpose

Box filters (also known as corner filters) are air-lifted aquarium filters, the oldest type of filtration still sold on the market, although their numbers have dwindled with the rise of more powerful filters, notably external filters. Still, the box/corner filter can be an excellent filter depending on the water volume and bio load of a tank.

  • Should be used in small tanks (5-25 gallons), at least per box filter.
  • These filters are perfect for slow-moving aquatic animals like axolotls, frogs, betta fish, gourmis, and angelfish. They are also recommended for fry.
  • These filters are very cheap, long-lasting (unless dropped and cracked), and dependable, especially in a power outage when they can be hooked up to battery-operated air-pumps.
  • Should not solely be used for high bio load tanks, like goldfish or turtle tanks.

Box filter that is designed to use an air stone.
Box filter that is designed to use an air stone.


In order to work, box filters need an airline tube and an air pump. That’s it--excluding the filter media, which is the material that is essential to filtration and is placed inside the filter.

Attach the airline tube to the input (smaller) tube that sticks out on the top of the filter, or if it’s a filter that requires an air stone, attach it to that air stone that sits at the bottom of the filter.

The other end of the airline tube is attached to the pump. Once plugged in, the air will pump through the airline and into the filter.


How It Works (Water Flow)

Like any other air-driven filter, rising bubbles in a long, narrow tunnel creates suction at the bottom, and it’s this suction that pulls water through the filter.

Typically, box filters have slits on top of the filter (often on the lid); this is where the water enters the filter, going through any media that is kept within it, and finally exiting the filter through the tunnel of bubbles. By traveling through the filter, debris is caught and biological filtration occurs as beneficial bacteria (invisible to the eye) grows on the surfaces of the media, thus providing clean water both visually and chemically.

How to Set It Up (Media Usage)

Unlike sponge and undergravel air filters, you can put anything in your filter as long as it allows proper water flow.

Mechanical media (which catches debris and keeps the tank looking clean) should be the first media water hits when entering the filter’s intake. Why? Because otherwise you’ll wind up with a dirty filter that needs changing frequently. You don’t want the debris to reach your other media, as the gunk will cover and clog the pores that help the beneficial bacteria colonize, and cleaning the media would have to be constant.

For box filters, mechanical filtration is usually filter floss (or pillow stuffing with no chemicals added). Sponge (or open foam cell) is another mechanical material, but you will eventually have to replace the foam as it begins to deteriorate over time, while floss is much cheaper and can conveniently fit as it’s just a clump of cotton-like material.

Chemical media is anything that will change the chemical balance of a tank, and should be placed after mechanical.

It was falsely believed that active carbon (charcoal) was essential for the filter, but it is now commonly used to remove medicine and Man-made toxins that are accidentally introduced to the tank’s water, like deodorant, soap, bleach, bug spray, and some metals, although this should also be accompanied by a large partial water change.

Cuttle-bone: found in any pet shop.
Cuttle-bone: found in any pet shop.

For increasing the calcium level of a tank (to bring up pH/GH that invertebrates and certain fish species need), the following can be included in the filter:

  • Pieces of Cuttlebone (for birds)
  • Pieces of Sea Shells
  • Crushed eggshells (placed in a filter bag--if wanting to keep separate from other media)
  • Pebbles/pieces of rock high in calcium

*To test for mineral content, place the rock in a container of white distilled vinegar; if bubbles fizz from the material, it releases dissolved calcium and will increase mineral content in the tank.

Peat Moss: this brand has no chemicals and can be found at nearly any hardware store. This I purchased at Menard's.
Peat Moss: this brand has no chemicals and can be found at nearly any hardware store. This I purchased at Menard's.

To bring down pH and make the water more acidic (for fish and plants of amazon origin, for example), tannin-leaching material should be included in the filter:

  • Peat Moss (chemical-free)
  • Crushed Almond Leaves
  • Small Pieces of Driftwood

To remove ammonia, they make sponges/media that absorb this natural toxin and are best for emergencies when a tank is still cycling.

Finally, Biological media does not change the chemistry of a tank, nor does it catch debris. In a sense, all media is biological media, as beneficial bacteria will grow on every surface within the filter. This bacteria eats up toxic waste produced by fish and uneaten food, turning it into a less harmful chemical that is only harmful at a certain amount (thus we perform water changes to reduce the amount). The point of having media that only serves as biological is to keep beneficial bacteria alive in the filter, as mechanical/chemical media are often replaced as they lose their mechanical and chemical efficiency over time.

Filter that uses all three filtration: Filter floss, pieces of egg shell, and ceramic rings and gravel.
Filter that uses all three filtration: Filter floss, pieces of eggshell, and ceramic rings and gravel.

The following are media for a low-to-average bio-load:

  • Gravel
  • Pebbles
  • Plastic Plants
  • Cut Up Straws
  • Anything Aquarium-Safe and Allows Water Flow

For a tank with a high bio-load (a lot of fish):

  • Ceramic Rings
  • PVC Shavings
  • Bio Balls
  • Anything Aquarium-Safe that is Porous/Rough in Texture


Maintenance should be performed on the box filter every 2-4 weeks to ensure water flow and efficiency in media.

For mechanical filtration, floss/sponges should be cleaned at least once a month to ensure water flow. The simplest, safest, and economical way of doing this is to take a small bucket of the aquarium water (never use tap or it will destroy your beneficial bacteria!) and squeeze the sponge/filter floss underwater several times to get rid of the dirt and debris, then once it loses its brown color, place it back into the filter.

If the filter floss or sponge looks worn out, it’s time to replace a portion of it. Typically, I replace half of my filter floss every two months. Always place the used floss/sponge in front of the intake slits, so water that enters the filter is first hit with biological filtration.

For chemical media, specifically regarding charcoal, ammonia absorbents, and tannin leaching media, fully replace every two weeks.

Media that raises pH like rock and cuttlebone dissolve their minerals slowly, so it can be many months until you’ll see a drop in pH. PH/GH levels should be tested once a month if using such media.

For biological media, once every month or so the entire filter should be taken out of the tank. Remove the mechanical and chemical media and whatever is left should be rinsed with aquarium water to get rid of built up gunk, which should be minimal if mechanical filtration is well-maintained. I often fill the filter with the biological media with aquarium water, swish the water around, and pour out the water to get rid of debris.

This process should be done quickly so the other media don’t dry out and lose their beneficial bacteria, or the other media should be placed in aquarium water while performing maintenance on the biological.

Tips/Helpful Additions

Don't Stuff, Simply Place

One mistake people make with these filters, aside from using tap water to rinse media, is that people will squeeze in as much media as they can, particularly stuffing floss or sponges to the point that water flow is greatly reduced, and thus making the filter weaker.

Instead, give the mechanical filtration some wiggle room. If the lid has pressure pushing against it by the media and there is no visible space on top or under the mechanical media, then there is too much.

Filter Media Bags

These make filter maintenance easier, especially filter media that need to be rinsed off and conveniently kept together, such as:

  • charcoal
  • peat moss
  • crushed egg shell (tiny fragments)
  • ammonia absorbents

These small and tall bags are meant for Hang On the Back (trickle) filters, but if filled 1/3rd of the way, can fit perfectly in a box filter.

Cheap cutoff pantyhose work just as well.

Suction Cups

Suction cups that are meant to hold airline tubes can help keep the box filter in place, especially if it doesn't hold heavy media like gravel or ceramic rings, as the filter will float thanks to the air pump. These suction cups are particularly useful to those with air stones, ensuring the air stone is kept at the bottom of the filter (the lower, the more suction).

These can be found in any aquarium store. For a cheaper amount in bulk, I bought over 20 meant to hold Christmas lights at a discount store for a few dollars. If the clamp is too wide to hold onto the airline tube securely, you can still create that hold by turning them, putting the airline at a slight angle. Also the more suction cups, the more stable the airline will be.

Gang Valves and T Valves

Gang valves help regulate air flow, increasing or decreasing the amount of air being pumped into the filter. These little single plastic gang valves are cheap and can be found in any aquarium store. Of course, these may not be necessary if the air pump already has a regulator for air flow.

T valves are convenient for using multiple airline tubes/filters/air stones from a single air pump. In one of my tanks, from a single air pump, I have a box filter, a DIY air filter, and an air wand running (for a species with a high bio load and in need of high oxygen in the water). I also use a T valve for a tank with a DIY filter and DIY algae scrubber.

When dividing airlines with T valves, I strongly suggest getting gang valves as well. There have been instances where if I didn't have a gang valve to increase airflow, one tube would simply not work.

T valves may not be necessary if the air pump has more than one outlet.

These two items are cost-effective, eliminating the need of purchasing multiple air pumps for multiple air-driven devices, although this means the more devices, the less air flow distributed from that single pump, so there is a limit.

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Comments 2 comments

Ann1Az2 profile image

Ann1Az2 23 months ago from Orange, Texas

I haven't used one of these filters in years, since the new outside filters came on the market. I do have one submersible filter that I keep in my 20 gallon tank and it has a charcoal filter and a bio filter in it. It works great. I used to have an outside pond that I made a homemade filter for and I used the plastic sponges, along with some volcanic rock. It worked great. There is really no end to what you can use for filter media.

alladin 4 months ago

thank u..

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