Keith Holmes says...

A bottom drain is an essential piece of equipment for a Koi pond as it allows for the continual removal of waste from the pond to the filter system. The best way to use a bottom drain is for supplying a gravity-fed filter system. The alternative is to use it as a means of dumping the pond water to waste periodically, but this results in any debris that has collected only being removed when the drain valve is opened. The problem with bottom drains is that they are very difficult to install in an existing pond. To install a bottom drain correctly, the drain unit (a 4in or 110mm pipe connected to a drain sump, if the pond is in excess of 1,000 gallons) needs to be installed in a concrete base, and to make this possible in an existing pond you need to empty the whole pond, remove the existing lining material and lay a new base, incorporating the new drain. If you already have a pond that was built without a bottom drain, which does not have gravity-fed filters, install a mid-feed through the side wall of the pond. A side feed should be positioned as low as possible and should be 4in (110mm) in diameter. Once installed you can use an elbow and a length of pipe to lower it to the bottom of the pond. It is best to use a side feed for a gravity-fed filter system, which will result in a certain amount of waste material being pulled from the bottom of the pond into the filters. A bottom drain is essential for a new pond; allow one per 4,000 gallons of water. Each drain should be located centrally and the base of the pond should be benched towards them. If you need more than one, divide the pond up into zones and repeat this process for each zone.