If your fish's tail doesn't open up after a few weeks in nice clean, warm water, you might try what I did. One of my VTs had that on his tail & fins, otherwise completely healthy. After a few weeks, I decided it wasn't going to go away (tried flaring him every day, no change). So, at one water change, I gently laid him on a smooth surface and rubbed the end of his tail/fins between my thumb & forefinger until the clamped part opened up. Now some pieces of the tail between the rays will come off, but those will grow back. When the tail grew back in about a week, it looked as good as new.