How to cure an unknown fish disease?

At this point i have no fish to save but i need to know how to get the disease out of the tank. Medicines never helped the fish. Every night i turned on the light three or four fish would be dead that were showing no signs of a disease. After a week the fish that were still alive were moved to a 10 gallon tank. I emptied the tank removed the gravel and sprayed it with bleach. After putting new gravel in, running the ornaments through the dishwasher on pots and pans in hopes that the hot water would kill the disease. I refilled the tank and let it run for a few days. In the 10 gallon tank the disease built itself up and killed like all but 2 fish. I moved them into the 125 gallon tank and everything seemed fine. After about a month i bought a couple of koi. After about 3 months of restarting the tank my fish started dieing of the same disease before. I decided to go with advice from a teacher and use this salt they sell at the local hardware store. It also didnt work. At this point i am out of ideas and out of fish.