71 is way too low...The room may be 71, but the water is usually 2 degrees cooler....You must have a heater.One easy way is to place the bowls in a clear rigid container, fill the container with water and heat the container...This way the water around the bowls is safely heated....Aim to keep it at 75-80...A quality, fully submersible heater is dirt cheap in the US and there is no excuse for not having one. Failure to heat them up is going to lead to a world of problems, including digestive problems, rot, bacterial infections and death...
PLEASE donot use any oils, garlic or otherwise in your bowls. I advise dumping the bowls and rinsing with vinegar many times to remove it. Rinse throughly with water.
You didnt mention the bowl size, but anything under a gallon must be replaced ever 24 hours with a 100 percent water change...This is non negotiable.
Good luck with your Bettas.