In that case, if possible and allowed by the place you're living, I would get a bigger tank. They're not that much more expensive than a smaller heater, which you would need for a smaller tank. Walmart has 10 gallon complete setups for about $30. That might be good for you for multiple reasons- you could cycle a 10 gallon tank, and then do just weekly partial water changes. The bigger the tank, the longer it takes to get dirty. Obviously, you do still need to clean it frequently, but not as frequently as a tiny tank. It also would be easier to maintain a stable temperature. Your betta might (probably would) be happier in a bigger tank with more room to move. Otherwise, you'd probably have to buy a smaller heater anyways- I really think that a 50 watt is too much, and the reason for your trouble. That heater might be made to keep the temp at 78-80*, but that would be for a bigger tank. My guess is the tank got too warm.