an anemone is rather sensitive to water parameters. That is one reason for when you read about putting them in a mature tank. As far as danger to fish, it depends on the type of anemone. Just because you have an anemone it does not mean that a clown will host in it. Also most will end up to large for a 20l. By the time an anemone reaches a LFS
most of the time they are to stressed to make a come back. I won't own an anemone for most of the reasons everyone has already posted. Here are a couple of pictures on my clowns hosting a plastic anemone and one hosting a featherduster.