Do you think it would be possible to upload some photos of your fish, alive or dead, with clear views of the "necrosis" areas? It sounds a lot like Fish TB. Ulcers and/or necrotic looking areas, curved spines, those are symptoms consistent with TB. Do you have fish that appear emaciated?
Be careful of cross contamination of any other tanks, whether TB or other pathogen this should be considered highly contagious.
Also, in case this is fish TB, you need to be very careful about putting your hands in the water if you have any broken skin at all. Fish TB can cause infection in humans, causing ulcers and granulomas at the site of cuts or broken skin on your hands or arms.
Also, please post your readings on ammonia, nitrite, nitrate. You use a liquid test kit, yes? What is the temperature in the tank