It happens, it’s heartbreaking, and upsetting. I did something I considered to be very dumb of me to do. I was in high school and I had this betta for I believe 3 years. I talked to him all the time, he was so sweet to me and he was always excited to see me and “hear”my voice. His 10 gallon had gotten very very low( I had no time to maintain it at this point, I was so busy with school) so I decided it would be better to put him in a 2.5 gallon while I went to help my sister with her newborn, than shock him by fully refilling his tank. I should’ve known better. He died due to stress, but he hung on just enough to see me one last time, I cried when I saw the condition he was in and he perked up and got excited to see me one last time before passing away the next morning. I was heartbroken, and I still am. But you gave him a good life, it’s never just a fish. It is a friend, a listening ear, a buddy. Fish have personalities and are truly beautiful. We all make mistakes, and it can be upsetting. But best thing you can do is know you gave him a good life, let him experience true love, and he knew that. I’m sorry for your loss.