I can definitely attest to betta fish being a little harder to care for than I thought.
My daughter bought a betta fish with her allowance, which we kept in a huge tank. However, we tend to keep the house a bit on the cold side and we turn the heat off at night. When the nights started getting colder, I noticed that the fish was looking a little less than well. I talked to my husband and we ordered a heater. However, when it came, I think I turned it too high too fast, and the water changed temperature too fast and shocked it, because it died.
I felt really bad for killing my daughter's fish, and have promised her a new one when we move (the tank is drained right now). And I will definitely have the tank preheated and ready next time, so there's no shock. I had looked up the right temperature, but hadn't thought about the extreme change being too much too fast. Poor Swimmy.