The purpose of the filter media is to provide a surface for bacteria to live and grow. Bacteria are either aerobic or anaerobic. In other words, requiring oxygen or not requiring oxygen. The bacteria that convert fish waste to nitrate are aerobic bacteria (requiring oxygen to work). I mention all this because when I talk about different media, one of the requirements for good bacteria is oxygen. Media have a big effect on the amount of oxygen available for the bacteria.


This is small gravel about the size and shape of peas, thus its name. It is very cheap and has a fair amount of surface area for the bacteria to populate. However, it tends to be heavy and clogs easily with fish waste and debris. When this happens it tends to channel. In other words, the water tends not to go around each stone but through channels created. Thus most of the gravel is not exposed to oxygen in the water, therefore, that bacteria dies. It is also fairly difficult to clean in order to remove these channels.


Similar to pea gravel only larger in size, which means it has less surface area. Because it is larger, the space between the rocks is larger, and does not clog up as easily as pea gravel. Although slightly easier to clean, it is still difficult to clean. Be careful that you don't use too much limestone gravel as this can cause water quality problems.


This is a fairly lightweight rock with a lot of crevasses, which in turn means good surface area. Due to its irregular shape it tends not to clog up. It is a fairly inexpensive media. Because of the crevasses in it, it is likely that it performs some denitrification (the conversion of nitrate to nitrogen gas by anaerobic bacteria). It doesn't have the same high surface area of some of the manmade media available today. However, if the filter size can be large enough this is a sure winner due to cost and maintenance. This media is underrated and overlooked. The reason behind this is that people trying to sell media can't make money on it because it is available at a low cost anywhere. The remaining media are manmade and as a result their use can become quite expensive.


Plastic open balls usually made of polyethylene. Normally these will not clog up or channel. Surface area is fair to good depending on size of ball and construction. These can have negative to positive buoyancy based on the material used in construction (negative buoyancy means it sinks, positive means it floats). These are typically used in trickle dry filters.


This media comes in a variety of shapes and sizes. When clean it has excellent surface area, but tends to clog quickly requiring frequent cleaning. Often it is used as a pre-filter for pumps. This can be used as either a prefilter or biological filter and often is used as both in the same application.


Usually made of polyester. Available in a multitude of sizes and thickness and densities from fine to coarse. It has very good surface area depending upon the coarseness of the fibers. This media looks similar to furnace filters. In time, it will clog up. Make sure the mat you use is fish safe.


This is either polyethylene or PVC ribbon. This thin, narrow ribbon has negative buoyancy and provides good surface area without clogging. The surface area per cubic foot is determined by how tightly this material is packed in the filter. It also must be contained in the filter.


Made of sintered porous glass. It is small porous cylinders that provide a very high surface area per cubic foot. It is a manmade lava rock type of product that is fairly expensive, but resists clogging. It can perform the denitrification function, as does lava rock.


These are typically used in the pre-filter to filter out the solids. They are available in just about any size. In addition to being a good mechanical filter, these are also good biological filters. They have good surface area. In the correct conditions they will clog up, as does any good mechanical filter. But when used as a biological filter, they will tend not to clog up. These are the major types of media. There are many other media being used today, but most of them are variations of these. One of these is the tubing and PVC pipes used to move water to and from the filter. These have very low surface area but can still do up to 40% of the biological filtration in a pond set-up. How is this possible with such a low surface area? This question is answered on the Filter page.