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The Corner Box Filter

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Prior to pump-powered filters (such as canister, HOB, overhead filters etc), the aquarium hobbyist had limited filter choices, and one of the more popular ones was the corner box filter.

Today, the corner box filter has fallen out of favor and has been replaced by more "modern" alternatives. I remember the days when we needed to run three corner box filters powered by two air-pumps just to maintain a stocked 40  gallon tank - a setup that even a small overhead filer can easily outperform.

But the corner box filter is still a great filter to own. It is one of the simplest, cheapest filters you can get, and still has many great uses.

The corner box filter is still one of the more common air-driven aquarium filters. It is basically a box that sits inside your aquarium that can contain various filter media and is often powered by an air pump.

 

A box filter can contain any filter media

How these types of filter work is pretty straightforward. An air stone is placed at the bottom of a tube within the filter box. The air stone pushes air and water out of a lift tube. This upward motion sucks dirty tank water into the holes on top of the filter box and through the filter media, cleaning the water in the process.

corner box filters are often air driven

Uses

Corner box filters usually have little filtering power (compared to modern alternatives).  Due to its small size, they can not hold a lot of media. And because they are typically air driven, they can not move a lot of water either.

Combine these factors (and that fact that they take up quite some space), you will end up with a filter that has limited uses.

But that does not mean that a corner box filter is rubbish. In fact, there are some very good uses for this simple filter.

Quarantine tanks:

The corner box filter is exceptional for quarantine tanks. In fact, this is my preferred filter for quarantine tanks.

Because you can use any media with these corner box filters, you can use some of the established filter media from your main tank's filtration system with these box filters. Using established filter media with the corner box filter helps in cycling the quarantine tank which can translate to better survival rates amongst the newly bought fish.

Hospital tanks:

The corner box filter is also great for use with hospital tanks (basically quarantine tanks with medication or treatment). Like mentioned previously, you can using media from an established tank to quickly cycle a hospital tank.

Breeding tanks:

Corner box filters can be used in breeding tanks. While sponge filters may be better in most situations, corner box filters are still very usable.

In the past, I kept a pair of jewel cichlids that started to show signs of spawning. Them, being the only fish in the tank, I installed a corner box filter and turned off the other filter; a canister filter, to prevent the soon-to-hatch fries from being sucked by the intake of the more powerful filter. 

Powered only by an air pump, using a corner box filter gave me some degree of confidence that the fries would be pretty safe. The clear design of my box filter also gave me additional peace of mind.

My jewel cichlid pair has spawned several times, and has produced a multitude of young, using only this setup. Watching these fish breed is truly one of the most rewarding experience a fish keeper can have.

A fairly colorful jewel cichlid pair becomes majestic in their breeding coloration. How they care for their brood, parents taking turns in protecting their spawn as the other one looks for food, moving their offspring around the tank from one hiding place to another - basically the entire breeding cycle is an amazing dance to behold.

As my experience has proven, if a sponge filter is unavailable, a corner box filter is a very viable alternative. The down side is that, it is quite weak, hence larger and more frequent water changes are needed to compensate for the filter's lack of the so called "umph".

Backup filter:

Finally, having a corner box filter is handy as backup filter for emergencies. If something unfortunate happens to your main filter, (such as leaking or busted motor) you need to fit as much biological media (forget the mechanical or chemical media for now) as you can into your corner box filter.  This will not only save your fish but also save colonies of nitrifying bacteria living in your biological media.

This biological media you rescued must be used again, as soon as you get your main filter system fixed. Using already established biological media will greatly help in re-establishing the newly repaired filter's nitrifying bacteria colonies.

Slow moving fish tanks:

I guess you can use several corner box filters for slow moving fish such as discus or angelfish, but for these fish, using something like a rain bar with a canister filter or simply using an under gravel filter might be better alternatives.

Disadvantages

Although a corner box filter is very useful in many situations, with other options available to the modern aquarist, I can not recommend this filter to be used as the primary filter - at most, it can be used to provide additional filtration. Here are some of the drawbacks one would encounter with the corner box filter...

Small bioload capacity

Since most corner box filters are air powered, they typically do not move a lot of water, limiting them to be used on smaller tank.

corner box filters are powered by air pumps like these

Takes up lots of space

But using box filters in smaller tanks can also be problematic because corner box filters need to be placed inside the tank, using up precious space.

Unsightly

Corner box filters can also be unsightly. They are a lot bigger than submersible pumps and are naturally harder to hide behind decor.

Messy maintenance

Cleaning and maintenance can be messy. If you have managed to hide your corner box filter behind decor, expect things to get a little bit messier come cleaning time.