RTG Wriggling / Tail Fin Issue

Hello everyone ! Hope someone can help me identify this problem. Bought a RTG around 1-2 weeks ago. After a while it was wriggling in a S pattern, I thought it was adapting to new environment so ignored it, but today i noticed the tail fin become like this.(See video below) What's the problem It's eating fine at the moment, 4 mealworms total with 1 mealworm fed every 1.5 -2hrs

Tank was cycled for 1.5mths with MP. Water in perfect condition since it was in the tank 0,0,0 for ammonia, nitrite nitrate. Ph at 6.9. WC with 1 bucket of water (works out to be around 2.5-5% of total water volume)everyday aged at least with 12 hrs with dechlorine. 422Tank with 3Ft Sump

Currently, I'm treating it with 0.5% salt (to kill parasites) ,heater set to 33degrees, powerhead turned on(got blind spot if it needs to rest) and full aeration. Any shifus can recommend what i can do to help it? Its still wriggling in a S pattern, im not sure if its biting its tail or what Should i continue 1 pail of age water with 0.5% salt per day or buy melafix?

Video here: (Paiseh not enough post counts to post link, paste link into browser and delete the space between youtube. and com)
youtube. com/watch?v=u-q7qS2mRgQ