I agree, mantids are predators and as such feed on other living animals but as responsible owners we either buy frozen (already dead mice) or have them killed humanely before feeding them to our predatory pets, this works both ways with mice as captive reptiles can be injured as easily as the mouse in the wrong circumstances.Not so with a fish, its very one sided.
The fish is a low order of life but it is way more advanced than a cricket or fly, to me this means it has a much more advanced nervous system and is feeling excruciating pain with every mouthful the mantis takes from it, compared to an invert (some thing to think about when we call ourselves animal lovers)
I can understand why some one might want to feed vertibrate meat to an invert, perhaps they feel it is a better food than an invert, which i dont think it is, to much fat compared to a much better balanced food such as another insect (which is naturally what makes up 99.9% of a mantids diet anyway) but not a live vertibrate, especially when we know that all we have to do is touch the mantids mouth parts with the vertibrate meat (dead already) and it will eat it, without inflicting pain and suffering to a higher order of life with a correspondingly more evolved nervous system.
A lizard will crush or shake and break the back of a mouse and a snake will envenomate or constrict its prey, not super quick (nature is cruel) but much much quicker than being slowly minced to death in the arms of a mantis, still these actions take place in the wild all the time but imo shouldnt have a place in animal husbandry because we recognise the cruelty. over...