Betta missing large piece of fin after two days - Causes?

[mookat]

Hi all! I just finished cycling a new 5 gallon Fluval Spec V. The tank is holding two nerite snails and a male betta, Tamaki. It has two anubias plants and some ground cover.

I had been keeping Tamaki in a hospital bowl since I got him from the LFS about three days prior to releasing him to the tank. All seemed to be going well - Stayed healthy, one small spot of damage to his top fin that he had since I got him, but seemed to be doing great. I released him into the tank on Saturday. Here is a video from then (note: The tank does have a little algae from light parameters being a bit funky):

This is his tail as of mid-day today:

What gives? Is he biting it? Could it be the tank decor? (felt these all over to check for rough spots). Could the snails be hurting him? Does fin rot move this quickly? I'm at a loss.

 

[Tyler_Fishman]

I searched the tank up, the filter for this tank seems like the culprit, sponge filters go best for Bettas with long flowly fins and the Caudal fin is the longest fin on fancy bettas typically. Nerite snails are very gentle and I highly doubt that they could even manage to touch a betta, I think he may have bit his tail at night, or possibly the gravel substrate may have damaged him possibly, rot is also a possibility but it usually taters all the fins not just one chunk. Betta fins heal quite fast in the right conditions, in about a month his Caudal fin should be completely restored