Mantis shrimp are some of our favorite invertibrates, and here's a little extra info that may help some of you decide.....the "hammerers", like the peacocks can be kept in tanks with fish as long as they are well fed, being specialized crustacean/bivalve/snail feeders (as well as among the most beautiful), and in larger tanks, more than one can be kept with the fish (while the average "cleanup crew" is deffinitely on the menu, keep in mind they BECOME the cleanup crew)
the spearers are a whole different story, as they are highly predatory on fish, and thus need their own tank (on the other hand, they will co-exist with many crabs and other crustaceans)
Here's a photo of our current mantis shrimp, "Blade", who's living in an old betta tank minus the dividers, with a small Eclipse bio-wheel filter and lots of caulerpa for nitrate control.
He's about 5" long (normal is up to 8") and a "spearer", and since we live in Florida, get them at really great prices (free) from the local bait shops where they come in mixed with the bait shrimp.
They are more than happy to give them away, due to liability is some poor tourist should get one in his bait bucket....the local nickname is "thumbsplitters"!!!**grin**