Fish that tend to shallow food whole, like lion's and groupers, most shelled animals are fine. There have been reports where people have seen lions 'suck a hermit right out of it's shell.' But it's very hard to digest the shell, so most snails stay safe. However shrimp are fair game
Fish with teeth, wrasses, puffers, triggers, hawkfish, keeping a CUC is pretty tough as they tend to like anything that moves. As mentioned by SR. a CUC is generally unnecessary, and most people fall back on them, and start to neglect general maintanence.