Hello, I've been struggling with my sick betta fish few weeks ago. His health progressed after clean water change and new filter. However, he is not fully recovered, and I noticed black part under his belly (where the pelvic fin is attached) Is this fin rot? I carefully check his past pictures, and he had some black since the day he came to my house. I think it just got darker and bigger? And one of his pectoral fin was half ripped off before he came. Several days ago, I went to LFS and bought some anacharis for him to rest. He liked the anacharis, but few days after, the tank started to smell like that LFS. Like some soil fishy smell. So I removed the anacharis and did 30% water change, but the smell is still there. Some of the rocks that were white changed the color to blackish white. I don't know if it is normal. Is this fin rot? I would like to make sure before any treatment. I had him for 1 and half month. Currently there is sponge filter, preset heater (78 degree), 5 gallon tank (water is about the half) I'm very cautious to do water change because he seems very stressed out whenever I do water change. Please let me know if this is fin rot or something else. Thank you so much for your help.