Nudibranches which are occasionally offered for sale, are extremely colourful. Unfortunately they require a very special diet to survive in an aquarium, and are therefore not well suited to tank life.
Many species will eat different kinds of corals, mostly soft corals like Sarcophyton and Zoa, but also Montipora for example, while other species will eat certain sponges or invertebrates.
The Nudibranch family includes the infamous Zoa- and Montipora eating snails, which are both small and easy overlooked.
These animals are often very poisonous, which their colour and lack of shell indicate.