Charles was my betta fish. I had him for about 3 days in a 5 gallon tank. He used a heater to keep his water nice and toasty between 78 and 80 degrees. His Top Fin 10 Power Filter was too strong. I came home to find his fin stuck in the filter's intake tube. I turned the filter off, and he was still just stuck in. I put him in cold water in case he was any bit alive, but I'm 99% sure he wasn't because he didn't respond to me getting him out of the filter or putting him in one of those betta cups.
For his funeral, I put some gravel in the betta cup he came in. I put in some water, and dropped him in. I poured lots of salt in the water to keep his remains from rotting and decaying so that some other organisms could feed off him. I sealed up the air holes with duct tape. I took him and buried his container in the back yard.