Last night my partners betta, Yul, passed away while I was changing the water. I don't know what happened at all. I scooped him out in his cup, sat him on the bench as I always do, thoroughly cleaned the entire tank, gravel, plants and filter, refilled the tank, made sure it was the right temperature, turned around to get Yul and he wasn't moving. I have no idea what happened to him, he seemed completely healthy, was eating properly, swimming as normal, as reactive as always when I scooped him out, then when I went to put him into the tank he'd died in his cup! I just wish I could know what happened.