I woke up this morning to find him lying on his side at the bottom of the tank. While I'm sad, I wasn't surprised because the day before, he was acting very strange. His hind end was limp and his tail fin was crumpled up. He was only moving his front half, and when i tried to feed him, he would just nudge it.

I plan on getting a new fish but I want to know killed my little friend so this doesn't happen again.

I kept him in a 5 gallon crescent tank that I cleaned monthly. It had a Tetra Whisper filter and a small Aqueon tank heater, with a few plants. I use Tetra EasyBalance on a weekly basis. I also kept a gallon of water pre-treated with water conditioner nearby to replace evaporated water. I fed him six days a week, and after his non-feed day, I would feed him a pea, to clean out his system.

I had just cleaned his tank earlier this week, as well. I used a gravel vacuum to siphon out most of the water and debris. I put the water into a brand new bucket that I had never been use for any other purpose. I used a paper towel to wipe off the algae. I changed the filter cartridge and rinsed off the filter itself, the plants and the ornament. While this was going on I kept my betta in a bag. I replaced about 30% water, pouring most of what was in the bucket back in. I put the bag with the fish in the water so the temps could equalize while everything in the tank settled. I put some of the tank water into the bag and waited about an hour and a half and let him back in the tank.

I'm sorry if this post is a bit long, but he seemed fine the other day, and I wanted to include every detail I could think of so I can hopefully figure out what went wrong. Thank you in advance to any insight you can provide.