Depending on the mantis shrimp that you want, the size of enclosure will vary. There are several
Commonly available species that can live in a 20 long. You just have to commit yourself to maintaining water quality which is important for mantis shrimp. I definitely
Wouldn't start with a peacock( o scyllarus ), they are very sensitive and prone to shell rot. Post on the mantis shrimp forum, you will get great input. But be aware that the likelihood of keeping tank mates with the mantis shrimp is small. But I love mine, she is awesome and you wouldn't regret going that route if you can get the equipment suitable for the setup