The fish would stress the mantis enough to kill the mantis (as they're in there the second you put him in during an already stressful time) or cause stess(shorten lifespan), possible malnutritrion from not coming out to feed enough, and reclusiveness. A smasher eats hard-shelled molluscs and crustaceans, not fish. It may attempt to threaten them into leaving it alone, but I doubt that would work. If it's a spearer, it would probably eat them. (Spearers are much less active and less seen than smashers, or so I've heard/read.)