Corner/Box Filters
How do these filters work? A corner/box filter is filled with carbon and filter floss and sits inside the aquarium. An air pump pulls water through the box and back into the water. This type of filter is known as mechanical and chemical filtration. There is a small amount of bacteria growth, but that is negated when the carbon and floss is changed.
Are there any problems with these filters? These filters are not that good for supporting a large aquarium or a high fish load, but they are fine for raising fry or a small number of fish. For best results, you can use gravel or other biomedia instead of carbon or floss. This will allow a small amount of bacteria growth.
Under Gravel Filters (UGF)
How do they work? There is a slotted plate that sits slightly raised at the bottom of the tank. Gravel is placed over this plate. At the ends of the plate are tubes that rise up into the tank. An airstone or water pump propels water up the tubes. So, what you have is a cycle of the water flowing through the gravel bed and under the filter plate, then up the riser tubes. By the water bringing oxygen into the gravel bed, it promotes the growth of beneficial bacteria. This is a very effective biological filter.
Are there any potential problems? The water quality in the tank can become poor because the gravel bed may become clogged due to the water flow pulling waste with it. Doing frequent water changes and gravel cleaning can avert this. A gravel vacuum or siphon hose can pull the waste out of the gravel bed while doing water changes. Some fish will dig into the gravel bed. If the plate is exposed, the growth of beneficial bacteria will be impaired.
