By: Douglas DuHamel
Selecting a filter for your aquarium can be difficult because there is a wide choice available. Some filters work well in small tanks and not in large ones. You also have to consider the type of fish in your tanks. You certainly cannot have a large power filter with tremendous suction in a tank full of babies. With all these factors to consider, you also have to think of prices. Filters can range anywhere from $5 up to over $100. In my article, I would like to describe the types of filters, how they work and the advantages/disadvantages of each one.
What are the basic types of filters? The most common types of filters available are corner/box, under gravel (UGF), sponge, power, bio-wheel, canister, fluidized bed, and trickle (wet-dry) 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.
How do they work? An external pump forces the water to flow through the sponge and then back into a water tube. Bacteria grows on the sponge due to the constant water flow. This is an internal form of biological filtration.
What are the benefits of these filters? They are inexpensive and easy to maintain. These filters are good to use in a tank where a gentle water flow is needed. They can support a large bioload, as long as the fish aren't too large.
What are the disadvantages? The sponges need to be rinsed on a regular basis because they get covered with waste.
How do these work? Power filters hang on the back of your tank and water is pulled into the filter, then pushed through a filter pad containing carbon. These provide both chemical and mechanical filtration. Some models also have a biological filtration pad. If your power filter does not have biological filtration, it can be used along with under gravel or sponge filters.
How do I maintain this filter? You must rinse out the pads on a regular basis. You will also need to replace the pads as the carbon weakens.
How do these work? It is a unit that hangs on the back of the tank where water is pulled into the unit by an impeller and then discharged back into the aquarium after filtration. As the water is pulled from the tank, it first flows through a filter pad where particles are removed. There is a layer of activated carbon inside the filter pad which helps remove impurities and keep the tank water clear. The water then moves through the bio-wheel where there is a large growth of bacteria. This filter is good for large fish loads.
How do I maintain this filter? You must rinse the filtration pads on a regular basis.
What are they? The canister is placed outside the tank and it's a tightly locked container. Water is drawn out of the aquarium through a hose and enters the canister where it passes through several compartments containing filtration media. The pump pushes the water back into the tank through another hose.
What are the advantages? You can configure the filtration media any way you want to. That is, you can use sponge along with bioballs and carbon. You can just use the sponge and carbon.
What are the disadvantages? They are expensive.
Fluidized Bed Filters
How do these work? It is a cylindrical pipe filled with sand and water is pumped through a filter, then through the sand. They have a massive bacteria growth.
What are the advantages? These filters can handle large volumes.
What are the disadvantages? You must use a pre-filter to remove waste and it must be kept clean. Fluidized bed filters should also be used along with a power or mechanical filter with a high flow rate.
Trickle (Wet-Dry) Filters
What are they? This filter is placed outside the aquarium and must be below the tank. Gravity forces the aquarium water to feed into the trickle filter where there is a bacteria bed. The bacteria is created by the use of a biowheel, bioballs or other biological medium. The trickling effect of the water plus the exposure to air creates a large bacteria growth that can support large fish loads.
What happens if I keep a reservoir of water in the filter? The biomedia must be kept out water. The mixture of air and a trickle of water is what creates the bacteria bed.
What are the disadvantages? It should be used in conjunction with a mechanical filter.
As you can see, there are many filters to choose from and it may be difficult to make a choice. My recommendation is to start with a lower priced filter that will do the job for your tank. You can always work your way up to an expensive one. Keep your old filter as a back-up in case you have an emergency or save it for the next tank you set up. Once you are hooked on the hobby, it's hard to stop at one tank!