You have electric leakage in your tank!!! Lateral line disease and hole in the head are usually caused by electricity leakage. Lack of vegetation could also play a major role in it. Water quality might not be a cruicial factor. Fish can withstand high level of nitrate, Po4, etc.. In saltwater, which is extremely high in alklinity? Ammonia and nitrite are extremely toxic! As long as your tank is fully cycled, your water should be fine for the fish. Of course if you want to maintain the health and long term survival of the fish you would want to create an ideal environment but in this case, water quality is not the problem.
ground the tank, feed vegetable that contains high vitamin C, stablize the environment would help. HTH!