So I made a shocking discovery! My Dad bought 2 red-tailed sharks about 3 months ago. We put them in my tank

Your other five fish did not kill your first redtail--the hiding one did. Red-tails are generally peaceful with other fish, but if bullied they will retaliate. They are very territorial and cannot be kept with their own kind, or even with other sharks that are not red-tails. There is nothing abnormal about your still living shark's hiding. They like to hide in small spaces and are sometimes reclusive around other fish. However, you red-tail should come out more often than it seems to. It may not be happy with the way your tank is set up (plants, lighting, etc.) or that may just be it's personality. Put in more things for it too hide in to make it happy, and research red-tailed sharks, in books or online. Adjust your tank to make it better for your shark after you research it. And next time you invest in new fish, look them up first to make sure they are compatible, and that you are not buying more than one of a territorial fish. If you really want more than one red-tail in a tank (I do not reccomend it!), put two in a large tank with thick foliage (live plants, not plastic ones), sand to burrow in, and LOTS of hiding places. Even if you do this, they will establish a pecking order. One will be brightly colored--more so than the other red-tail--and will chase the other away from the food at feeding time. Research your fish next time to avoid the same mistakes.