Box or corner filters, are the oldest type of filtration in home aquaria. Using a corner filter and a handful of media from an established filter can quickly and inexpensively set up a hospital tank. Because it has a gentle flow rate, it is also a great choice for fry tanks. The biggest downside of the corner filter is the space it takes, and unsightliness in the aquarium.
1. Lee's Discard-A-Filter
Lee's new disposable filter comes with carbon, zeolite, and a sponge, all in a compact disposable unit. After a month, discard the entire unit and replace. The small size is great for mini-aquariums.
2. Lee Economy
The Lee Economy model does not use an airstone, and therefore does not have the high flow rate seen in the triple flow design. However, you can't beat the price. At under two dollars virtually anyone can afford to keep one on hand to quickly set up a small hospital tank in a pinch.3. Penn Plax Clear Free
More compact than other corner filters, the Clear Free is easy to install and maintain. Unfortunately it lacks the flow rate of the corner filters that use airstones, and it's price is higher than any corner filter on the market.