Hi, everyone! I've had my sassy lil sweetheart, Stitch, since November 4th, and I'm almost positive he was born on August 30th because on the cup I got him in, it said that. His tail was a lot shorter when I first got him compared to a week ago.
I had Stitch in a bowl... I'm thinking about a half gallon - (maybe full?) just until I had enough money to buy him a tank with a filter. I know it wasn't good to have him in a bowl, but I loved him as soon as I saw him he was so sweet and I wanted to rescue him from that small cup!
Sooooo, a week ago I finally saved up enough to get him a 2.5 gallon tank with a light and a filter called the Aqueon MiniBow. I also got him gravel for the first time. In his bowl, there was nothing but him and the water. I changed the water every 2 days, and since having the MiniBow I have changed the water 3 times.
All was good until on Friday. I always watch him swim. Right before I went to shower, his tail/caudal fin was beautiful and intact... I went to take a shower, when I returned I saw a small hole was on his tail... Just after being out of my room for about an hour!!! I returned the tank the next day and got a new one of the same kind because the filter was spitting out junk instead of keeping the water clean and it was cloudy after 1 day. Seemed weird to me because his small bowl never even got that bad that quick. After the new tank, the hole healed.
Then, on Sunday, I was watching him again. Went to the bathroom, came back seriously a minute later, a HUGE chunk of his tail was gone!???? I freaked out and searched the filter, water, but no signs of his tail anywhere... I figured he bit it off!!!! There is no signs of fin rot... (from what I know about fin rot).. no blackness. He still is swimming like crazy and is the little spitfire I have always known... What could this be? I don't want to lose my baby. :[ I have had bettas before when I was younger and they all lived in bowls for a little over a year. (I didn't know that that wasn't the proper care.) And upon researching, I felt terrible and wanted my lil guy around for a long time... and as soon as I got the bigger tank with a filter that's when it went downhill! I don't get it.

Is it ammonia? Fin rot? Or just him biting it? There's tiny shreds on his anal fin it seems too...
I've used water conditioner so far. I really don't have much money, so I don't have ammonia test kits and all of that.. Just needed some insight on what this could be.. Thanks in advance. <3
Stitch when I first got him:
Stitch when I first got the tank:
Some pics of his tail after the big chunk is gone:
Here's a video of him flaring, since in the pics of when his tail was normal they're kinda clamped and you can't see the difference of when the part of it was missing:
https://www.facebook.com/photo.php?v=249835781833902&set=vb.100004224462590&type=2&theater