They are probably dying from either temp, salinity, or pH shock. Remember you scrubed your rock then plunged it into your tank. I doubt you acclimated your rock and the inhabitants on it. There will be a certain amount of die off just from that. Believe me there are many more that will survive. Your tank is in its infancy, sudden death is commonplace. Don't worry about the gorillas, be thankful they are dying!
