Hello, I am a new betta fish owner and while doing a bit of cleaning in my betta's tank today I noticed his back tail fin looks tattered and pieces are missing. It wasn't like that yesterday and I'm pretty worried.

This is what his fin looked like a few days ago:

Looking back on pictures from when I first got him about a month ago his fins look very different.
Is this fin rot? What can I do?
He is currently in a 3 gallon tank with a heater and filter. He also has live plants and a coconut shell cave.

This is what his fin looked like a few days ago:

Looking back on pictures from when I first got him about a month ago his fins look very different.

Is this fin rot? What can I do?
He is currently in a 3 gallon tank with a heater and filter. He also has live plants and a coconut shell cave.