My slider refuses to eat, its eyes looked somehow abnormal, and it gets lazy and not moving, but when I come near or try to change the water it moves normally, I asked the pets shopkeeper and he told me to put a certain antibiotic in the water for 4 days, I did with no progress, so I continued for 10 days, I think its eyes get better, but it's still lazy and refuse to eat anything, I tried to feed it with my hand but no way. Now I rinse it weekly with water and salt, and I put it in the sunlight every day.
This continued for about 3 months now, today I held it in my hand and found the shell is very soft and weak.
What can I do? please help, my slider is dying

Can I do anything to feed it? or at least can I put something in the water to compensate the weakness of its shell?
I'm not expert with sliders, so please help me