Depending on the type of filter system you have, cleaning can be done by back-flushing of the filter chamber or removing the filter media by hand. Never use tap water to clean your bio-filter. We cannot emphasize enough the importance of maintaining an adequate population of bacteria in the filter media. Koi ponds should ideally be four to five feet deep, but need to be at least three feet deep. Deeper ponds are best for maintenance of a consistent temperature throughout the year and protection against common koi predators such as raccoons and herons.
Many cities require permits for certain depths of digging.
Please contact your local government for specific regulations. The only way a koi would be capable of eating a goldfish is if the koi's mouth can accomodate the goldfish's entire body. Koi feed by repeatedly sucking at the surface of the water, so that a very small goldfish living with a very large koi may not be able to outswim the koi's vaccuum-like mouth. In most ponds, such size disparity is not present. A different problem arises in the case of small fantail goldfish.