Internal aquarium filters are used inside the aquarium. They have built-in motors and cling to the wet glass using plastic suction cups. These filters work best when positioned near the substrate. Working properly, they are efficient at cleaning the water in smaller tanks of 30 gallons or less. In larger tanks they make great secondary filters. However, sometimes filters fail.
Water Stops Moving Through the Filter
If the motor is humming but no water is moving, the motor works but the filter itself has shut down. The way to know if this happened is to see whether the surface of the water in the tank is still. It's possible that the filter needs to be cleaned out and may work fine afterward. First, remove and clean the filter, and put back together properly.
Filter Rattles When Turned On
Normally an internal filter will rattle just a bit when first turned on, as the water is introduced and the mechanisms begin to run. If this rattle doesn't go away within a few seconds, something may be broken inside. Unplug the filter and give it a light shake to see if anything rattles inside with all the water drained out. This could mean that the carbon filter has broken and pieces of carbon are loose.
Dirty Parts Inside the Filter
When you change the filter media, examine the filter itself to make sure the parts are clean enough to draw water. Unless you do so frequently, the filter can clog, forcing the motor to work harder. The impeller is usually first to become clogged with algae and fish waste, so clean it using aquarium water and a brush. Tap water often contains chlorine, which kills the beneficial bacteria inside the filter.
A Crack in the Facade
Check the filter for cracks, dents and broken bits. If the hose plug is broken, the filter is damaged and the air hose won't work. If this is the case, you'll have to replace the filter, as it is essentially permanently broken. Internal mechanisms such as the impeller shaft and bearings may be broken. Replacement's usually cheaper than repair.
Suction Cups Missing
Sometimes the suction cups on the back of an internal filter stick to the glass when you remove the filter for cleaning, and they get ripped from the filter. You can usually put them back on by bending the plastic connectors to make the cups fit inside the slots on the back of the filter. This happenstance may not affect the filtering function or it may disable filter function, depending on how the filter rests. Your aquarium's not safe if the filter's not secure, though.
A Word About Cleaning
Filters are breeding grounds for beneficial bacteria, which are necessary for healthy water. If you clean the filter and media too well, you'll destroy these good bacteria. Rinse the filter only to the point that the worst of the algae is removed and no further. If the mechanisms are jammed up, an old toothbrush scrubs them clean enough to work, but be sure to just rinse the algae in the filter box and not remove it entirely.