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

Ah can leh. Here you go =D

[Spider88]

Bro, very bad water condition. Are there other fishes in the tank that might attack your aro? This looks likes rot fin disease. You may get some rot fin medication from aqua shop. WC and add salt. Change ro wash your media might help a little.

its tail rot...plz check water n filter n change em....
n add tail rot medications n add salt