Aquatic animals that moderately need to be kept in an cycled aquarium tank. The animals put into this difficulty are usually are relatively hardy, but may have some requirements that make them moderately difficult for some to keep. This could mean that they will not tolerate fluctuations in their tank environment (temperature, pH, cleanliness, etc...), need careful consideration with tankmates (i.e.: timid and can't be kept with boisterous fish, or territorial and risk causing harm to any other kept with them), need specific foods, and other such things, including mixtures of the mentioned potential requirements.
These would be best kept by those with some experience in fishkeeping, or by beginners after extensive research.
Go here to take a look at a list of the aquatic animals put into this difficulty: Fish Difficulty - Moderate.