So, Salazar as my tail biter... Recently I noticed a little bit of black on his fin which I thought might have been rot. I started doing more frequent water changes and dosing him with AQ salt.
Well, this morning I cupped him and changed his water. I rinsed out the tank, his decorations, etc. I put clean water in the tank and added Nutrafin Aqua+ water conditioner and 1.25 tsp of AQ salt (he was in a 2.5 gallon tank). Once his tank was all set up, I put him back in, like I've done every other time I changed his water, and he started darting around all over the place. First, I thought he was exploring because I moved his decorations around. But then he started swimming on his side, as though he was having trouble. So I cupped him to put him in more shallow water so he could get air, but he died not even a minute after the water change.
I think it might be something in the water, but this was his third water change since I came home from school and nothing like this had happened with the other two changes and Severus, my other betta, seems absolutely fine. He's gotten new water twice since I brought him home--once when I first set up his tank at home and the second when I had to empty his tank in order to move it to clear off some counter space so my mom had more room to cook for Christmas Eve. He's fine.
Do you guys have any idea why he might have died so suddenly?