Finrot and fungus on betta?

Hi i have a cycled tank (community) 60 litres my tank has been set up fr approx 2 years, i do water changes every other week along with gravel syphoning, i bought a betta yesterday, which was fine in the shop but within hours his fins are frayed and hav brown tips, i know its fin rot, i did a 40% water change straight away and put aquarium salt in, but what do i do next? Ma tank water is clean and good condition ( i have succesfully bred and raised 50 kribensis in it before) please help!