What do you mean by watch out?
THE things you need for a mantis tank: (simple)
-A tank 6~100 gallon depending on species.
-standard lighting, not strong, mantis shrimps do not like bright lights.
-sand 1/2 the tank if it is a spearer.
- some live rock to provide cover and to hold PVC if you want
-PVC
-I have macro algae not mandatory.
-heater if it gets colder, a chiller vice versa
-all the other stuff like carbon, sponge, pump, and refractometer.
I have a peacock mantis and a P. Ciliata
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THe peacock is a smasher and the thing you have to watch out is the peacock getting mad and cracking the glass. So you need a big tank for this guy. He hits the glass once in a while, but he didnt crack it yet... ALso peacocks are messy eaters. THey bury their food and the tank gets dirty fast.
THe P. Ciliata is a spearer I just got from this forum. Anyways they dig, so you need to put a lot of sand in and they just stay in there until there is food. Then it comes out and stabs it. You need to worry about the boredom it causes. Actually they are both boring, but the spearer is more boring because it just camps out. At least the Peacock beats the crap out of the other inverts.
* the main thing you need to worry about is money. I feed mine every other day and smasher food is about 1-3 dollars a day and spearer food is 5 dollars every 2 days.
That is about it.
~Abe