i may be able to help if your looking to find the mantis a new home...and willing to ship. mantis would be only one in a 20g long
You could build a 10 gallon for these really cheap! 10 gallon tank, 10 lbs of sand, 1 nice chunk of live rock, and a power head! You're ready to go! Best lighting would be something really dim, so moon lights would be great for them. You're talking maybe $150?
Pretty sure I mentioned that depending on species it can injure or kill the Mandarin. I was more commenting on most marine aquarium owners trap and kill Mantis Shrimp. This is a shame because they are actually a very cool pet in a species only tank. I have owned two different species so far. My current one I've had for over a year now.