50 gallon tank fish disappearing?

It sounds like your iridescent sharks are eating the smaller fish. People often tell you what you want to hear on fish forums because they want to get high rankings so its really best to research through books and reputable websites. Also, beware of petshops that sell you monster fish with a promise that you can return it when it gets to big - they know its probably going to die before you get around to returning it and they can just pocket the money. Plus, some people are able to return the fish before it dies so you may be buying someone else's problem fish. I would return the sharks now and possibly find a different fishstore.