Undergravel filtration uses the sand and coarse coral to filter the water. Bacteria grow within the sand and multiply as oxygen passes through. A well maintained undergravel system is an effeceint way of breaking down ammonia within your aquarium.

Water is drawn through the sand, along the under gravel plate which is a platic tray with holes in. It is then forced up through the pipes and into the powerhead (which drives the whole system). Undergravel filters have been very popular in the past, however never and more efficient ways have been invented.

The main drawback of this type of filtration is the amount of extra equipment needed, as well as if left unattended the sand can become clogged, reducing the flow of oxygen to vital bacteria. Also there is no room to add UV sterilisers or Nitrifying equipment to the system.

However this said the undergravel filter is a cheap way of maintaining your system, and with the addition of live sand and live rock can become a stable environment. Combined with a canister filter (instead of a powerhead, a input from a canister is used) extra equipment can be added.