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A basic question facing a hobbyists is what filter to use for an aquarium. The answer tends to be different for each hobbyist and possibly even each tank a hobbyist owns. Indeed, it would take more than a column or two to discuss the possible answers to the question of the "best" filter. I want to explain two common aquarium filters: hang-on-the-back filters (referred to as power filters) and the canister filter. I'm hopeful this information will help you decide which of these filters would be best for your next aquarium.
The hang-on-the-back filter ("hang-on" for short) has been around in one form or another since the early 1960's. The general components of a hang-on filter are the box that hangs on the tank, the intake tube through which water flows from the aquarium into the filter box, and the discharge area that is commonly a dam over which water flows back into the aquarium. Thus, the water path is fairly simple - water is drawn up the intake tube, into the box and flows back to the aquarium by way of the overflow. The filtration media is placed in the filter box in the path of water. This way the water is forced to flow through the filter material before going back to the tank. In general, the filtration consists of a pad or block of some kind of material for mechanical filtration, followed by activated carbon.
Up until the 1980's, most hang-on filters operated by siphon action. In this style of filter, a pump forced water through a discharge tube (rather than over an overflow) out of the filter box. This created a siphon action, causing water to flow from the aquarium through the intake tube to the filter box (you had to start the siphon by filling the intake tube with water, holding your finger over one end and quickly placing it in the filter box). While this style provided good mechanical filtration because the water had to flow through the filter material, it had a major drawback - maintaining the siphon. If the water level in the aquarium dropped too low, the siphon would break. The pump would then empty the filter box, resulting in no filtration and a lot of noise as the pump continued to spin while there was no water to pump. Fish would sometimes knock the intake tube, breaking the siphon. In addition, changing the filter media involved taking the filter off the tank, which could be messy.
In the 1980's, a new style of hang-on filter was introduced.The basic design is still used today. In this model, the intake tube is positioned in the filter right above an impeller, thereby eliminating the siphon action. Water is continuously pumped (pulled) into the filter box (unless the water level in the tank gets really low). Because the filter box sits a little higher than the lip of the aquarium, water can flow back into the aquarium by gravity. The filter media are in the path of the water, which is flowing upward in the filter box. This represents a compromise between convenience and filtration efficiency. These filters are very convenient compared to the older generation of hang-on filters, but they don't filter as efficiently. This is because as the filter pad clogs, the water cannot pass through it as quickly even though the pump is pumping the same amount of water. The result is a buildup of the water in the filter box. If the water is not given a place to go, it can overflow the filter box onto the floor, which is why all hang-on filters have a second overflow at a higher level than the first that allows water to flow back to the aquarium instead of onto the floor. The consequence of this is that this water is not filtered because it has bypassed the filter pad.
The main point to remember about hang-on filters is that they almost always pump water, but the water may not be filtered. In general, the hang-on filter is for someone seeking an easy-to-use, effective filter - someone willing to perform some routine maintenance, such as changing the filter pad. Fortunately, changing the filter pad is easy for most hang-on filters. The old pad is lifted out of the filter and thrown away and a new pad is taken out of the box or bag and slid into the filter. In most cases, the filter does not have to be turned off. How often the pad will need to be changed depends on how much food is given to the fish and the number of fish in the tank. Every once in awhile, a more thorough cleaning of the filter should be done. This entails cleaning the inside of the intake tube, cleaning the impeder and the area where it sits, and cleaning the filter box. Canister filters were developed in the late 1970's and early 1980's. The basic idea behind a canister filter was that no water should be able to bypass the filter media. In this way, the hobbyist is assured that all water exiting the unit is filtered. This is accomplished by having a sealed unit that usually sits under the aquarium. The water exits the aquarium through tubing into the canister. There is another hose that will direct water from the filter back into the aquarium. Depending on the brand, the pump sits either under the canister or on top of it. The pump is used to move the water from the canister back into the aquarium, not to pull the water out of the aquarium into the canister. Inside the canister is filter media which can be arranged in a variety of ways depending on the design of the filter and the kind of fish being kept. Most commonly, the first media provides coarse mechanical filtration. Next is usually activated carbon, then possibly media that is used as a substrate for biological filtration