get ready to do a water change. I can't imagine a spider doing tons of damage. more likely the fish killed it when it hit the water. unless it was one of them African Bird Eating spiders, then you are screwed. Seriously, sounds like something else is going on. It could be that you stocked too many snails at the start of your tank and they starved. follow what yote said and if things are off, do a water change. if not, sit back and enjoy your tank. DON'T try to fish out your cora beauty it will get over the Ich in a week or two on its own
-Doc