A hang on tank or H.O.T. filter can provide mechanical, biological and chemical filtration in an aquarium. It is called a hang on tank filter because it hangs on the back wall of the aquarium, with the filter itself external to the tank. An intake tube extends inside the aquarium pulling water into the body of the filter, where it passes through various mediums before being returned to the tank.
A common hang on tank filter consists of a plastic rectangular compartment segmented by an insert in which a filter bag is kept. A small magnetic driven impeller motor pulls water into the filter where it first passes through a foam pad. This removes large debris like uneaten food, waste, or bits of plant matter, thereby providing mechanical filtration.
Next the water passes through a filter bag that normally holds activated carbon. Activated carbon pulls organic pollutants from the water, further purifying it. This provides chemical filtration.
Finally, carbon and the foam pads provide a home for biological bacteria that helps purify the water naturally, thereby providing biological filtration.
In theory a filter that provides all three types of filtration: mechanical, chemical and biological, should be quite effective, and these filters do fulfill a purpose, but they also have limitations. For one, because of their relatively small size, even the largest hang on tank filter needs frequent maintenance. Particularly the foam pads need to be rinsed regularly, and this disturbs the biological bacteria living there. Even the debris in the foam pads can inhibit bacterial colonies. Also the carbon must be thrown out periodically, and with it, its bacterial colonies. Although this positive bacteria lives elsewhere in the tank as well, this isn't an ideal situation.
As a result Marineland designed a hang on tank filter called a BioWheel. The BioWheel has a paper-like accordion drum that sits in the outflow of the filter. The water rushing out of the filter turns the drum in the same fashion a riverboat wheel turns. The papered drum is host to the bacterial colonies, and never needs replacing or cleaning. The rest of the filter is designed traditionally. Now when the foam pads that provide mechanical filtration are rinsed, or when carbon is discarded, the filter does not lose its biological filtering power because the majority of the bacterial colonies live on the permanent BioWheel drum.
Although the BioWheel addressed the problem of improved biological filtration, hang on tank filters lack the capacity necessary for highly effective mechanical filtration. For this most aquarists turn to more expensive canister filters.
Marineland, again, decided to bridge this gap with another innovation that combines a hang on and canister filter. They call it a H.O.T. Magnum. The H.O.T. Magnum is a hang on filter of a completely different design. In essence it is a miniature canister filter designed to hang on the tank. The H.O.T. Magnum is more robust than traditional hang on tank filters, and is simple to maintain. It can also be used as a powerful vacuum. However, no hang on tank filter can compete with a standard canister filter, due to a standard canister's sheer size.
That doesn't mean hang on tank filters don't have their place. The rule of thumb is the more filtration the better. For this reason most aquarists use a variety of filters for different purposes. A moderate sized to large tank with a UGF or under gravel filter, hang on tank, and a standard canister is a common set up. The UGF keeps debris pulled out of sight until it can be vacuumed, the hang on tank might provide convenient chemical filtration that can be swapped often and easily, and the canister would provide highly efficient mechanical and biological filtration.
Whether or not a hang on tank filter is right for you will depend on the size of the tank, the amount of fish, and many other factors. In almost all cases a hang on tank filter comes in handy as either a primary filter or a redundant one. Keeping an extra hang on tank on hand for a hospital tank is also a good use for these trusty little filters.