I used a 5w Aqueon mini heater. Learning very quickly that it didn't have an internal temperature controller, I had to constantly turn it off when the temperature was over 80. So I found this mini heater while looking for a replacement. absolutely love it. My betta tank lasts at 78 max and 76 min from what I've seen. It has a status light at the top that's dim enough not to be intrusive.
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