Scavengers such as catfish are carnivorous fish. They can be really useful in garden ponds because they will consume excess food and keep the waters healthier than if the decaying food was left to rot. They also clear up mosquito larvae which is good news for humans (although goldfish too will eat these) and other aquatic insects such as caddis fly. Most scavenging fish are hard to spot because they skulk at the base of pools vacuuming up the debris. Catfish are distinctive for their whiskers although their colour is a les than exciting murky brown grey and therefore extremely hard to spot. The professional advice is to introduce a catfish with caution. They can be aggressive and their scavenging is not restricted to unwanted food and insects. They are partial to small fry and can completely destroy any chance of successful fish breeding in a small pond. They will grow up to 30 centimetres long and will eat larger fish when fully grown.