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What Happened to the Box/Corner Filter for Fish Tanks?

Marie is an aquarium aficionado and loves taking care of her turtles and frogs.

Box filters now seem to be passé in the aquarium world. Learn more about how these filters work, their advantages and disadvantages, and why you might still want one.

Box filters now seem to be passé in the aquarium world. Learn more about how these filters work, their advantages and disadvantages, and why you might still want one.

In the beginning of home aquariums, box filters (also known as corner filters) were the only means of convenient filtration for aquarists. But in the past decades, new gadgets have been developed in order to provide other means of filtration, such as other internal filters like the sponge filter, undergravel filter, and the now ever-so-popular external filters like the power (waterfall, HOB) filter.

Each type of filter has its own unique design for filtering out pollutants in the water. No type of filter is perfect, however, as there are always some drawbacks to the kind you choose. But why is the box filter, the oldest means of aquarium filtration, nearly extinct?

Where Are the Box Filters?

Walk into any chain store’s aquarium section and you’ll find no box filters on the shelf. In fact, search for them online and you’ll find they are almost always on sale, as if stores can’t get rid of them. Only the local aquarium store may have a few box filters on the shelf, but most likely you’ll have to buy yours online after some heavy searching.

So, what’s the deal? I will go over what the box filter is, its pros and cons, and my own theory as to why it is disappearing. I will contrast the box filter with the power filter because it is the most popular filter and I have no experience with the other types.

This diagram shows how a box filter works.

This diagram shows how a box filter works.

What Is a Box Filter?

I prefer the name “box filter” rather than corner filter, because I tend to think of corner filters as the triangular ones designed to be in a corner, while there are round and square box filters that are meant to be placed anywhere. So it’s like all corner filters are box filters, but not all box filters are corner filters, even though all of them are almost always placed in a corner anyway. It’s just my preference of name. And the filter, no matter what shape, is like a box, containing its entire media inside.

The box filter is air-powered and one of the simplest filters in design and materials. It is a clear plastic box (or whatever shape) with slits on its side and/or top where the intake takes place.

Charcoal is the most common filter media, and it is compacted by aquarium filter floss (cotton-like) placed on top and/or bottom, which serves as mechanical filtration. In order to make the filter work, it needs an air pump motor and a clear tube that is meant for air stones; the air rises up through the filter, while water is sucked into the slits through the mechanical and chemical media.

It is very simple when you see it in action.

Why I Love the Box Filter

Even though the design has been supplanted by modern filter types, there are several advantages to the old box filter.

Cheap in Every Way

The filter itself can be a few dollars to $10; the higher price range is meant to filter larger tanks. Once you buy the filter, the pump, and the tubing, all you will ever need to buy is the floss and what other media you use. Active carbon and floss are often sold in bulk, so you can buy a year’s worth of floss or more for just $8. Carbon or other media are often pricier, but once you buy it you are set for a long time.

The power filter by itself is more expensive because a motor is a part of it; you have everything you need from the get-go, but this also means more money. Instead of buying floss and other media separately and in bulk, power filters use cartridges that combine the floss and carbon into one, with the addition of plastic borders that keep everything in place. They also have different sizes for different size filters, so you are very limited in your choices. And because they come with plastic sides, sown-like pockets of floss, and carbon conveniently in place, it is more expensive.

In the end, a power filter will cost you at least five times the amount of a box filter in media.

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Perfect for Slow-Moving Animals

A power filter creates a current underwater, so if you have slow-moving fish or small, weak critters, a power filter can make life for your animals stressful. Of course the bigger the tank, the less likely they’ll have to put up with the current. They do make adjustable power filters to control the strength of the water flow, but these are a lot more expensive than the preset ones.

The box filter only disrupts the surface, and even this current isn’t that strong. If a box filter is placed in a small aquarium, and surface turbulence is an issue, merely place floating or tall décor in the path of the ripples so your air-breathing animals have some calm water to come up to. Remember that the current on top does contribute to aerating the water, so don’t cut off most of the turbulence.

Box filters create a weak current at the surface, making them safe even for fry. This particular filter is a corner filter, as you can see from its shape.

Box filters create a weak current at the surface, making them safe even for fry. This particular filter is a corner filter, as you can see from its shape.

Safe

It’s no wonder so many people say box filters are perfect for fry and breeding tanks. There’s no current underwater so the smallest fry can swim freely, and the filter intake isn’t as strong as a power filter. There are plenty of horror stories of little fish getting sucked into the power filter’s intake, but the box filter isn’t as strong.

While the filter does have slits big enough for the tiniest fry or African dwarf frog legs to get into, combined with the low intake, the filter floss behind the slits prevents animals from getting past. You may find your shrimp pulling out some of the floss through the slits, but they will stop once they realize there’s no food in the floss.

Lasting

Unless the manufacturer uses the thinnest layer of plastic possible, the box filter lasts. I have one that’s been in use for 20 years, and besides a chipped-off piece on the bubbler tube (where bubbles come out), it’s held up.

Because power filters are much more complex than the box filter with moving parts and a connected motor, if it stops working then odds are you’ll have to buy a new one. If the motor dies, you can’t just replace that part. If your aquarium has hard water and the hardness accumulates inside it, it will eventually clog and there may not be a way to get rid of the built-up deposits deep inside.

Easy to Clean

As mentioned above, the power filter can become ineffective in time, which is why it needs to be regularly cleaned. I clean my box filters maybe once a year, and it’s easy because you have access to everything. The only part that requires a pipe brush is the air intake and bubbler tube, but because of the tube sizes, it would take years for accumulation to build up to the point of slowing down the air flow, if at all. It’s a quick simple clean, and convenient to do so when performing a media change when the box filter has to come out of the aquarium.

Can Be Used During Power Outages

This goes for all air powered filters. Battery-operated motors are typically used for air stones while transporting fish, but it can also pump air into your box filter during an emergency when you have no electricity, so your tank will continue filtering and your fish won't be jeopardized. I think everyone with an aquarium should at least have one battery-operated motor and an air driven filter as a safety measure. Only a generator will keep other filters working during a power outage.

And out of the three types of air filters, the box filter is the best option in case of an emergency, because you can take your filter's biologically established media and place them in the box filter (in order to keep the nitrogen cycle going--the main point of having a filter). Can't do that with a sponge filter and would be challenging to do so with an undergravel one.

Variety in Media Options

As I've already established, the box filter differs from the other air driven types due to its unique design. While sponge filters only have sponge material as filter media, and undergravel filters use, well, gravel, the box filter is the best choice if there are particular media you want to use.

There's (mechanical) filter floss, sponges, (chemical) activated carbon, zeolite, phosphate absorbents, peat, crushed coral, (biological) gravel, ceramic rings, lava rocks: whatever you want, you have the option. Sponge filters and undergravel filters can still provide excellent biological filtration (as well as mechanical for sponge), but if looking for an air powered filter where you can have all three methods of filtration (mechanical, chemical, and biological), the box filter is it.

Filter floss, frog moss (pH experiment), ceramic rings, and gravel.

Filter floss, frog moss (pH experiment), ceramic rings, and gravel.

Criticisms of the Box Filter

No filter is perfect. Here are some of the common complaints about the box filter and my thoughts about each criticism.

Insufficient

I have read that some people say the box filter is ineffective. The power filter is stronger and filters more water per hour than the box filter.

This is true; power filters are stronger. But to say the box filter is ineffective is false. I’ve used box filters all my life and they do a great job sustaining my tanks.

Like any filter, not all sizes are meant for the same size tank. There are box filters for 10 gallons and then some for 40 gallons. The average, old fashion box filter (made by Lustar for example) is best suited for 5- to 25-gallon tanks, so it would only be ineffective if placed in a much larger tank. Or, if placed in a tank with a high bio load (amount of fish waste).

Power filters can (depending on their size and the tank size) manage higher bio loads, while box filters are better suited for low to medium bio loads. So it's possible that a box filter may not be enough in a 20-gallon if it's housing goldfish or large schools of fish - anything that produces a high amount of waste.

If wanting to use box filters for a tank larger than 25 gallons or with a high bioload, I'd suggest using two box filters in opposite corners. Not only does this ensure you're getting enough surface agitation and filtration in longer tanks, but it makes partial media changes safer, as you will always have one filter with all of its beneficial bacteria.

Box filters tend to be considered unsightly, and while they do spoil the illusion of an all-natural tank, there are things you can do to conceal them.

Box filters tend to be considered unsightly, and while they do spoil the illusion of an all-natural tank, there are things you can do to conceal them.

Unsightly

I’ve read some people think the box filter is ugly and ruins the look of the aquarium. I guess that’s just a matter of opinion. I don’t think they’re beautiful, but ugly? A heater isn’t attractive, but it serves its purpose and I don’t mind the view of it either.

Personally, I think this is a petty reason not to use box filters. I understand that if you’re going for a natural-looking atmosphere, the filter does ruin the illusion. If this is truly a concern, merely place a fake plant in front of it and it’s hardly noticeable.

This is why I prefer (and miss) Lustar’s green filters because the greenish tone of the plastic blends in with the background better. Recent box filters like Lee’s are just clear, so the floss remains white and sticks out a little bit (before it eventually turns brown or gray). But that won