Fin Rot

[fish_food]

Tank size: 20L
Temp: 23 oC
Ammonia: 0-0.25 ppm (I use seachem prime which may give a slightly higher ammonia count)
pH: 7.6+
Medicine: Cephalexin 500g
Problems: Fin rot, turned on side
 
 
My bubble eye goldfish has severe fin rot, which started because his tank mate had been nibbling on his fins.
I initially quarantined him to a small hospital tank on which I performed daily water changes.  I tried a salt treatment, which didn't seem to do much as bits of his fins just keep falling off.  
 
Two days ago, he took a turn for the worse and developed a kink/bend and started floating on his side.  After googling, I found that this is a tell-tale sign of nitrate poisoning so I did an immediate water change and added 500g/1 pill of the human antibiotic, cephalexin to the water.  He is super sensitive to ammonia due to poisoning in the past.  He has also lost his appetite, and I'm trying to hand feed him.
 
I don't know what to do with this persistent fin rot and his current condition.

 

[watertown28]

There is no other symptoms besides the 2 you listed?
 
When you say a "kink", is that along the spine? If that is the case, your fish could have scoliosis, which to my understanding there is nothing you can do. If your fish does have that that extra stress will play into not being able to get the fin rot under control.
 
Also, the floating could be swim bladder. If you could take a pic of your fish that would help greatly.

 

[fish_food]

Thankyou for the reply, the kink was like this picture.

 
He'd shown the same kink when he was suffering from ammonia poisoning, however this time, he wasn't able to recover.  I buried him this morning.  
Although I hope that it won't be so, what should I do in the future if I encounter persistent fin rot again?