Mantis
Mantis
I have several mantis in my 125. I also have 6 chromis, 2 royal gramas, a mandarin, a 1 inch tiger goby, 5 brittle stars, 250 blue legs hermits, shellfish, and snails. THe mantis have not bothered anything yet. I even still have some barnacles, a favorite mantis food.
The mantis in the 125 are relatively small. I also had three adults, two red and one green. I saw them within 2 days of getting the rock they were living in and they were easy to catch and are in their own ten gallon tank. The litlle ones are still in the 125 and after more then four months have not bothered anything else. I can here them clicking all the time, but rarely see them. Once in a while I will see one looking out at me and they enjoy a little piece of raw shrimp every now and then.
Anyway, unless you have adults, which wold be easy to spot I would look for another source for the loss of your fish.