I would also think there was something dead in your tank. Turn over all your rocks, dig through your substrate if you have to, but I think there's definitely something dead and rotting in there, especially being that you have ammonia.
If all else fails, AmQuel+ is the best thing ever for getting rid of a ammonia, nitrites, and nitrates. I've used it with inverts and corals and it's never harmed anything. It's fantastic, IMO.