In my humble opinion, a fish being eaten by a Praying Mantis is no different than a fish being eaten by a Mantis Shrimp or any other aquatic animal except that it would probably feel more pain being eaten in the water than out of it, because if it's out of the water it will probably be focusing on trying to breathe.I rear fishes and my favourites are the Silver Arowana and the Oscar. I've observed them consuming feeder fish and they definitely do not kill their prey instantly, just like the Praying Mantis. In fact, I think, if one were to observe how an Oscar or an Arowana kill prey too big for them to swallow, it would definitely look more violent than a Praying Mantis eating fish.
And if it is said that this doesn't happen in the wild, I don't take it as a valid argument unless someone living 24/7 in the wild has confirmed it. I've seen a video of a Mantis actually catching fish from the water on YouTube, not hand-fed. I would post the link but I'm afraid to start any argument. Now if a Mantis has the capability to actually do that, then it's possible that they might just eat fishes in the wild.
Thoughts from an Asian who keeps Mantids and Fishes and loves them equally!