Your going to do this: your going to put your turtle out of the water but on a moist envioroment with paper towels and next your going to put allot of a fatty antibiotic ointemment (for people like B-panthene plus) on your turtle´s tail. I can see by the pics that your turtle´s body wants to regect the dying tissue, it just needs some help doing so. Your turtle HAS salvation. Dont try to desinfect with any agressive stuff like alcool or betadine, and keep your turtle clean. He needs to be out of the water for like 2 days, then put in water to drink and stuff for 1 hour then take him back again for another day. If it goes well your turt should regect that part of the tail in less then one week. Dont try to pull it or anything, its a natural process.