My betta is in a 5 gallon and I keep him at 80 degrees, they are much much much more active at warmer temp, I would definitely recommend getting a nano heater! you will be able to see much more personality, brighter colors and they wont clamp their fins like the ones in those sad and crule little bowls. I used to have some shrimp in my tank with him but he found out that he could flush them out of the foliage and they made a satisfiying crunch!! so like jpappy789 said: keep the tank densly planted!
I actually have a nano heater in there now, but it doesn't keep the tank nearly warm enough. I keep my house at 58 at night and 64 during the day in the winter and the tank is around 66-68 degrees. I so wish my nano was enough. I am bummed that I wasted money on it.
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