Just take some water out of the tank and keep an eye on the temp. If you have a thermometer stick that in. They prefer 75-80 but as long as the temp doesn't drop below 70 he should be fine. My beta used to live in water that was like 68 all the time. He was just a little sluggish and I'm sure that kind of temp isn't good long term so try to get him his own tank asap. A nice little 3-5 gallon will do nicely. That is what I'm planning to do. Honestly betas do better when they are alone or have a couple of cory cats to keep them company. That's my experience.
As far as keeping the temp steady, just don't put him near a heater or a keep him by a window. Any sunlight or draft will cause the temp to change too fast.
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0.5 Gallon - Orange/Purple Beta Splendens(Caramel) 
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