First, I would suggest holding off on any iodine supplements until you are sure you haven't saturated the water with it. Secondly, popping sounds at night are a symptom of a mantis shrimp. How do the bodies of the fish look when you find them? If they are torn apart and partially eaten, especially if there is nothing else in the tank, then you more than likely have a mantis shrimp somewhere, and it is preying on your additions. You may need to sit up one night after you have turned the tank lights off and see if you can locate the source of the popping sound. If it turns out to be a mantis shrimp, DON'T try to grab it with your hands as it can easily hurt you. Get a trap from your LFS and bait it. If you do capture one, I would suggest trying to sell him, as opposed to flushing it down your toilet, as they do fetch a pretty decent price to people who would like them in a species tank.