Ripped Fin?

My roommate and I keep several bettas and up until this point, minus one jumper before we knew anything about bettas, they've all been very healthy and active.

Well, recently I went home on our school's month-long break and left the bettas in the care of my roommate. I assumed she'd take good care of them, but when I got back all of their tanks were filthy and they were all listless and wouldn't eat. (Apparently she got 'bored of cleaning their tanks... !) It's been a week and they're looking alright, except for one, our oldest and largest betta.

He is eating fine and swimming around alright, though a bit more lethargically than before, but his anal fin is torn. It's a clean rip from the bottom straight up to his body, seperating it into two pieces. There's no curling or any other sign of damage to his fins, just one clean rip. Is this a sign of something serious, or does it sound like he might have been flaring too aggressively?

This is a really bad picture (after I messed up the flash once on him, he refused to come out of hiding) and I apologize, but it still gives a pretty good view of the ripped fin.

That has happened to my bettas before. I would have a mirror behind the tank and he would flare so much that it ripped or in my divided 2.5g tank when they were first in it they spilt their fin. So most likly your roommate put a mirror behind its tank or the room mate put all the tanks next to each other and they flared to much. So what size tanks are they in?

I'm glad to hear it's nothing too unusual. The only reason I was concerned was because this particular fish isn't much of a flaring fish- if he sees another betta he lunges at the glass, but never really flares.

He's in a tank that's about 1.5 gallons; it's an irregular size so I'm never really sure.

Thanks, durbkat.