If your id is right, it most likely isn't going to be a threat to fish. There are multiple people who have mantises as large as 4-5 inches in tanks with fish. smashers do not prey on fish in the wild. The biggest risk is a fish coming too close to it's burrow and getting a warning blow. You may loose a few snails, but you will gain a very behaviorally interesting and intellegent pet.
The fish are not 100% safe, but they are more than likely not going to be killed by a mantis this small and reclusive.
~Katana~