A hang-on filter is external to the aquarium and hangs on the side or back. There’s an intake tube that runs deep into the aquarium and a motor that draws water into the filter. The water is then run through various filters before being returned to the aquarium tank.
- It can filter mechanically, chemically, and biologically
- Because the motor is small, it’s a very quiet filter
- It’s probably going to be the least expensive of your options
- Due to the design, it cannot meet the needs of large tanks without other systems (i.e. protein skimmers, denitrator coils, refugium, etc.)
- Frequent maintenance will disrupt necessary bacteria colonies.
Commonly, the water is drawn from the intake tube to foam pad of some type. This traps uneaten food, plant matter, waste, and other debris. This step will remove the largest particles. This is the mechanical filtration.
Just a note, make sure your intake tube has a grill or vent to prevent fish from being sucked in. I kept a tiny frog in a freshwater tank and one day, it disappeared. It baffled me until I cleaned out my filter and I saw bits of the frog.
Next the water is run through a filter bag, often containing activated carbon. The carbon binds with toxins and pulls the organic wastes from the water. This is the chemical filtration.
Finally, bacteria forms on both the foam pad and filter bag (or activated carbon). The bacteria breaks down ammonia (a waste product of animals like carbon dioxide) into nitrite and then into nitrates, which can be used by any live plants you have in the tank.
This is a straight-forward design and is simple to understand and operate. However, you also need to provide frequent maintenance. You should rinse out the foam pad weekly of debris. Also, you need to change the filter bag once a month.
This has the unwanted side-effect of disrupting the bacteria colonies that live on the foam pads and filter bags. Although the bacteria lives in other parts of the aquarium and will eventually repopulate your filter system, this is less than ideal.
One way to get around this is not to replace all of your filter media all at once. For example, if you have a foam pad to catch large particles, cut it in half and rinse out one side at a time. If you have activated carbon bags, put the new bag with the old for a week, before disposing the old bag.
The rule of thumb for filters is more is better. When a hang-on filter reaches its limit, the next step up is the canister filter.