Hi, i have a walking catfish that ive had for 2 years. Every few mounth he was developing small pink growth on his head that disappeared a few weeks after with medical treatment. about 2 weeks ago he developped them again. 2 days ago he stopped showing interest in food and today he started to show nerological sighs as iirraticly swimming, swimming upside down,twitching his body while swimming.
I wanted to ask for help in diagnosting and treating this disease. He is in a small tank about 50 L water p are 0,,0,10.

Sadly, the answer to your walking catfish's problems is contained with this https://www.seriouslyfish.com/species/clarias-batrachus/ - he requires, and certainly after 2 years, a tank of more than 40x the water volume he's currently in... at least 2,300 litres.

I know that he needs a huge tank but what it have t do with the current illness?

A fish succumbs to illness, and ultimately death, when it's not being kept in line with its requirements. The link I gave you details the requirements of the walking catfish; keeping such a fish in a tank of the size you're keeping him in is very cruel. Take a read of these articles which ought to explain the situation better:
http://injaf.org/the-think-tank/how-t ... nditions-can-affect-fish/
http://injaf.org/the-think-tank/tankbusters/
https://www.fishkeeping.co.uk/articles_23/ethical-fishkeeping.htm

Were a "shelter" for fish. all of our fish are rescued.. the only reason that he is in this tank is that he attacked other fish. We were looking for a bigger tank for him.

today he started to swim in a circle , like he is chasing his tail, he's doing it almost all day without stopping.