Thank you all for your condolances. At least they will swim together in the big sky-pond.
I think the tumor was old age. In my experiences with bettas, when they reach a certain age their health starts to rapidly decline. They stop eating and blow bubbles through their gills. Before Cupid died she had a seisure of some kind. She began to writhe slowly, touching her nose to her tail. She would stop breathing. Then she would recover and act like nothing happened. It was so hard to watch, she looked to be in great pain. Now she is in a pain-free place. <3