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I am not an expert and new to this hobby. but to me, it looks like fin rot. my betta recently contracted the same though I had maintained my tank pretty well thus far. any how, I am using aquarium salt and frequent partial water changes and have seen some improvements. though he still got and is not out of the woods yet.
would recommend the same. but since yours seems to be a community tank, I will suggest you to isolate your infected pet to a quarantine tank. if you don't have a 1 or 1/2 gallon quarantine tank, I will suggest you to get one. worth it since these kind of things happen on a frequent basis.
I am a "newbie" too, but I have had a betta for 6 years (he died at 6&1/2). But once he did come down with fin rot and sad to say, thats what it looks like your fish has. I would definately put the sick fish in a quarantine tank until he has returned to normal. Get a medicine specifically designed to treat fin rot/fungus(for fish) and add it to the water (following the directions on the package) I know that treating the water with Maracyn and performing frequent water changes helped the problem when my betta had fin rot.