Hi and welcome to the forum
If the fish's tail has been eroding away since November last year, then it is either poor water quality or a bacterial infection. If you used the medications as directed and the problem is still occurring, it could be a drug resistant bacterial infection.
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I would stop adding medications (including salt) and do a 75% water change and gravel clean the substrate every day for a couple of weeks. If the tails gets worse during that time you can take the fish to a fish vet and get them to take samples of the tail and send it off for testing to find out what species of bacteria is causing the problem.
Alternatively, you can try a Tetracycline/ Doxycycline based anti-biotic and treat him for at least 2 weeks and see if it helps.
Option 3 is to put the fish into a container of Methylene Blue and then move him into a new container with clean water and hope for the best.
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You should try to vary the fish's diet and include a vitamin supplement and foods with anti-oxidants.
Look for a fish vitamin supplement and add it to his food every day. If you can't find a fish vitamin then get a reptile or bird supplement and use it 6 days a week but do not use it on the 7th day.
You can add juice from orange and purple coloured fruits/ veges to his food. Some good foods to juice include apricots, orange squash, pumpkin, orange and purple carrots and blueberries. These foods can help boost his immune system and might help him fight off the infection.
Sick fish should be fed a varied diet 3-5 times per day so they have lots of nutrients throughout the day and it will help them regain some energy and put on some condition. You can feed him frozen (but defrosted) bloodworms, brineshrimp, daphnia, prawn/ shrimp, fish. You can also feed small live or dry insects like aphids, live brineshrimp & daphnia, small flies, mosquitoes and mozzie larvae, ants and ant eggs. Basically most small non toxic insects that have been caught by hand and are free of chemicals.