Haha, my catfish eats snails straight up, shell and all. Not really sure how that's digested, but he's never spit the shell back up so.... yeah. The fish will probably deshell it himself though - some of my other fish suck it out through the front. Or flowerhorns can probably just crunch it.
Spiders I have no idea. They're good bugs for the ecosystem though, so I personally would leave them, haha. And besides, while you're trying to catch them, you might get a spider bite, and those hurtttt like heck. For all you know, they might bite your fish and leave an ugly looking welt for a while - not exactly appetizing.