Betta Fish: Help- fin rot?

The first picture was 8 months ago when we adopted him. He naturally turned mostly blue and red over time, and looked healthy and happy. The second and third pictures were today(not the best pictures)

Against my best judgement, my boyfriend and I put snails from a brackish pond into his old tank, which we now know was really dumb. We didn’t really think it would be harmful and thought it would add life to the tank I guess. They basically destroyed the ecosystem and today we got our betta a new tank, clean water, conditioned it, etc. but he has been lethargic recently, his tail/fins look frayed, and he’s turning black. Fin rot? What can we add to treat? We have read about almond leaves, Rooibos tea, aquarium salt and more. We are new fish owners and don’t know the next steps. Please help

-10 gallon tank with heater and filter
-we have current tank at 79 degrees F, old tank with the snails was unheated unfortunately
-old unheated tank had the snails in it for maybe 2 months.
-New heated tank bought today
-water changes were every 2-4 weeks
-feeding a few betta fish pellets a day
-no tank mates anymore
-few live plants like bamboo, and a couple tank hideouts from the pet store