Good god that tank need maintaining with a gravel vac, high nitrates anything thing above 050 wan't fetching down with a water change, when did he last maintain the filter, plus high nitrite reading thats want fetching down with a water change, how many gallons is the tank, plus full stock list of fish of how many and which kind.
The fish have a bacterial infection you will need a bacteria med.
Don't alter the water stats to fast though you will send them into shock, but that tank needs some prompt attention as the fish are sick.