Bubbles can appear from your fish tank filter for a variety of reasons, and it's not always a bad sign. If your filter is producing an abundance of bubbles, such as enough to cover the tank's surface, it is not working at peak performance. Once the reason has been diagnosed, the fix usually is not too difficult.
Bubbles in the Filter
Most fish tank filters are air-driven and operate from a pump. It stands to reason some bubbling will occur in the cascading return. These bubbles are usually large and disappear quickly. Another bubbling may occur when the return water from the filter hits the surface tension of the tank water. Raising the water level to meet the outflow will eliminate this effervesce. Another simple fix for too much splash is to lower the water flow setting on your filter. An abundance of air seepage through a sand or sponge filter may mean blockage. The clogged media, pushes air through pockets and into the backwash. If that happens, it's time to change your filter media. If the gradation and weave pores in your box or outside filter wear thin, this too can cause an abundance of bubbles in your aquarium.
Potential Bubble Sources
An air pump issue can cause bubbles through the filter system. A worn diaphragm in your air pump may allow bypass air through your system. Many filters have repair kits containing appropriate sized diaphragms. A bad impeller, causing continuous loss of prime, may be the source of filter bubbles. If the pump impeller is bad, it may need to be replaced. Most likely the impeller is clogged and can be cleared with a small brush. Overfeeding your fish will cause bubbling when the filter backwash cascades back into your aquarium.