I went into a store to buy a 25W heater on Tuesday and they refused to sell me one. The reason being that I wanted to use it in an acrylic tank. They said in such a tank, the heater is 100% guarenteed to melt the tank, kill the fish and cause a possible fire hazard. They said, if I buy it, it is at my total risk. They said that several keep Bettas, even when it gets very cold locally without any problems. For now, that is what I have decided to do.