There is a new generation of hang on back (HOB) filters for aquariums that are being offered by the major aquarium filter manufacturers, and they are definitely an improvement in the technology. With the earlier versions of aquarium HOB filters there was always a problem when aquarium hobbyists went to clean or change out the filter cartridges in that they would also get rid of the better part of the bacterial colony that was driving the nitrogen cycle — the natural biological activity at the heart of a successful aquarium. Part of this problem was definitely the manufacturers’ doing, aided by the local fish stores, because they wanted to sell replacement filter cartridges.
The new types of aquarium HOB filters have been very cleverly designed, in that they have a lot of plastic that stays in the filter housing when the filter cartridge is cleaned or replaced. In this way, the beneficial bacteria of the nitrogen cycle are disturbed as little as possible. So, with these new aquarium HOB filters it is possible to do as frequent filter cartridge cleaning, or replacement, as the hobbyist wants, which will add greatly to the clarity of the aquarium. At the same time, the biological balance of the bacteria will be disturbed only minimally.
Another innovative improvement in the new aquarium HOB filters is that the pump/motor that moves the water from the aquarium, through the aquarium filter and back into the aquarium, is located in a position on the aquarium HOB filter so that it will start up as long as there is enough water in the aquarium (the operative limitation seems to be about 1.5 to 2 inches below the top of the aquarium). This means that, unlike the earlier generations of aquarium HOB filters where losing power meant that the water pump needed to be primed before it would pump, when the power comes back on with the new aquarium HOB filters they just start right up. With the earlier types of aquarium HOB filters, if the power went off and then back on without the hobbyist knowing it, often the pump would run dry and burn out. Having the water pump so that it is always in the water solves this problem. It’s really nice when a new aquarium product is truly an improvement.