My sister's opal betta, Squid, which she purchased from Petco in January, has just passed away. He was physically very healthy, and alone in a heated and filtered 10 gallon with many plants and hides. Initially, we noticed that his fins were being torn. We assumed it was stress biting, but we were clueless as to the responsible stimuli. Nothing in the room had been altered. In the matter of hours, his tail was being ripped shorter and shorter (nothing sharp in the terrarium) and it turned a pinkish-red (potentially from the trauma to the area?). Last night, we found him lying dead at the bottom of the aquarium. He had been eating regularly the day before, and was free of pineconing and fin rot/melt symptoms. My sister does not own a water testing kit, but maintains the tank by doing partial water changes with treated tap water. He was super active and the pinnacle of health for the past five months, under the same routines, until this incident. Do any of you have an explanation for Squid's mysterious and unexpected death?