Harlequin Rasbora Caudal Fin Damage

I have a 13 gallon with a male betta, 5 harlequin rasboras, a mystery snail, and 2 nerite snails. I added the rasboras 5 days ago and have been watching very carefully (nothing else to do during this stay at home order) to make sure my male betta was ok with his new tank mates. There have been no issues at all. He swims with them a lot of the time. Sometimes sits on a plant or in one of the rock hidey-holes. No aggression whatsoever.

I did my weekly water change this afternoon. Just a little while ago, I walked by the tank and looked in to see where Elton and Bernie (nerites) were and noticed one of my harlequins has most of the upper half of his caudal fin missing (pictures attached).

I have no idea what happened to him. I've been watching closely again and still see nothing to indicate a cause.

Is he going to get sick? What can I do to help him heal?

 

2 more pictures

 

Is it possible that during your water change you spooked him and he tore it on something? I’ve had fish get caught in some crazy places while doing a wc

 

That is entirely possible. I'll have to be more careful next time. I'm most concerned with what to do now to help him heal.

 

That is entirely possible. I'll have to be more careful next time. I'm most concerned with what to do now to help him heal.

Keep an eye on him. If he did just damage it, clean water and low stress should be all he needs to heal. Even if it were fin rot, that should still heal with clean water low stress.
But if it starts to rot away more, then think about something like a bacterial infection that may need more help to heal.

I think he should be fine with good water quality.

 

Perfect! Thank you! I'll keep a close watch.