I'd treat with a whitespot treatment tbh. My sister had an outbreak and treated and they're all gone. It's such a shame you lost your CT red. Sometimes stress when they first come to you can lower their immune system leaving them vulnerable to allsorts. My Betta was at deaths door for the first part of the week when he came to me. Just laying there on the gravel not eating. Then he got fin rot as a result. He's all better now thank god.
Indian Almond leaf is very good and has healing properties in it. One in their tanks would be a good idea.
Thank you. Every bit of adice is good as I am on edge with my 2 remaining Bettas. Although they are extremely healthy right now, I fear for that day that something might happen. I guess its been a bad run for me. I lost my oldest swordtail( old age) 2 bettas (ich) and a pleco(unknown). So I am having a bit of rough time with my fish, and all my water conditions are fine. I used the API master test kit to check....I guess bad luck sometimes strikes in this hobby